Add eth_capabilities method for routing capability discovery

src/eth/capabilities.yaml

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+- name: eth_capabilities
+  summary: Returns effective capabilities for routing decisions.
+  description: |
+    Returns information about the data available on this node, including the
+    current head block and retention policies for each resource type. This is
+    useful for RPC routers to determine which historical queries a node can serve.
+  params: []
+  result:
+    name: Capabilities
+    schema:
+      $ref: '#/components/schemas/EthCapabilities'
+  examples:
+    - name: eth_capabilities example
+      params: []
+      result:
+        name: Capabilities
+        value:
+          head:
+            blockNumber: '0x13f8e3a'
+            blockHash: '0xd4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3'
+          state:
+            disabled: false
+            oldestBlock: '0x12ab34'
+            deleteStrategy:
+              type: window
+              retentionBlocks: 90000
+          tx:
+            disabled: false
+            oldestBlock: '0xf00aa'
+            deleteStrategy:
+              type: window
+              retentionBlocks: 2350000
+          logs:
+            disabled: false
+            oldestBlock: '0xf00aa'
+            deleteStrategy:
+              type: window
+              retentionBlocks: 2350000
+          receipts:
+            disabled: false
+            oldestBlock: '0xf00aa'
+            deleteStrategy:
+              type: window
+              retentionBlocks: 2350000
+          blocks:
+            disabled: false
+            oldestBlock: '0x0'
+            deleteStrategy:
+              type: none
+          stateproofs:
+            disabled: false
+            oldestBlock: '0x120000'
+            deleteStrategy:
+              type: window
+              retentionBlocks: 90000

src/schemas/capabilities.yaml

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+EthCapabilities:
+  title: Effective routing capabilities
+  description: Describes the data available on the node. Each resource reports the oldest available block and the deletion strategy, allowing RPC routers to determine which historical queries a node can serve.
+  type: object
+  additionalProperties: false
+  required:
+    - head
+    - state
+    - tx
+    - logs
+    - receipts
+    - blocks
+    - stateproofs
+  properties:
+    head:
+      title: Current head block
+      description: The latest block known to the node, useful for determining sync status.
+      $ref: '#/components/schemas/EthCapabilitiesHead'
+    state:
+      description: "Account and contract state including balances, nonces, code, and storage. Determines the available range for historical state queries. Affects: eth_getBalance, eth_getCode, eth_getStorageAt, eth_getTransactionCount, eth_call, eth_estimateGas, eth_createAccessList."
+      $ref: '#/components/schemas/EthCapabilitiesEffectiveResource'
+    tx:
+      description: "Transaction data and lookup indices. Determines the available range for transaction lookups by hash or block position. Affects: eth_getTransactionByHash, eth_getTransactionByBlockHashAndIndex, eth_getTransactionByBlockNumberAndIndex."
+      $ref: '#/components/schemas/EthCapabilitiesEffectiveResource'
+    logs:
+      description: "Event logs emitted by smart contracts. Determines the available block range for log queries and filters. Affects: eth_getLogs, eth_getFilterLogs, eth_getFilterChanges, eth_newFilter."
+      $ref: '#/components/schemas/EthCapabilitiesEffectiveResource'
+    receipts:
+      description: "Transaction execution receipts including status, gas used, and logs bloom. Determines the available range for receipt lookups. Affects: eth_getTransactionReceipt, eth_getBlockReceipts."
+      $ref: '#/components/schemas/EthCapabilitiesEffectiveResource'
+    blocks:
+      description: "Block headers and bodies including transactions and uncles. Determines the available range for block retrieval. Affects: eth_getBlockByHash, eth_getBlockByNumber, eth_getBlockTransactionCountByHash, eth_getBlockTransactionCountByNumber, eth_getUncleCountByBlockHash, eth_getUncleCountByBlockNumber."
+      $ref: '#/components/schemas/EthCapabilitiesEffectiveResource'
+    stateproofs:
+      description: "Merkle proofs for account and storage state. Determines the available range for proof generation. Affects: eth_getProof."
+      $ref: '#/components/schemas/EthCapabilitiesEffectiveResource'
+
+EthCapabilitiesEffectiveResource:
+  title: Effective resource capability
+  type: object
+  additionalProperties: false
+  required:
+    - disabled
+    - oldestBlock
+    - deleteStrategy
+  properties:
+    disabled:
+      type: boolean
+    oldestBlock:
+      $ref: '#/components/schemas/uint'
+    deleteStrategy:
+      $ref: '#/components/schemas/EthCapabilitiesDeleteStrategy'
+
+EthCapabilitiesHead:
+  title: Current head block
+  type: object
+  additionalProperties: false
+  required:
+    - blockNumber
+    - blockHash
+  properties:
+    blockNumber:
+      title: Head block number
+      $ref: '#/components/schemas/uint'
+    blockHash:
+      title: Head block hash
+      $ref: '#/components/schemas/hash32'
+
+EthCapabilitiesDeleteStrategy:
+  title: Delete strategy
+  oneOf:
+    - title: No deletion
+      type: object
+      additionalProperties: false
+      required:
+        - type
+      properties:
+        type:
+          type: string
+          enum:
+            - none
+    - title: Sliding window deletion
+      type: object
+      additionalProperties: false
+      required:
+        - type
+        - retentionBlocks
+      properties:
+        type:
+          type: string
+          enum:
+            - window
+        retentionBlocks:
+          type: integer
+          minimum: 0

tests/eth_capabilities/get-capabilities-disabled-stateproofs.io

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+// Returns effective capabilities when state proofs are unavailable.
+>> {"jsonrpc":"2.0","id":1,"method":"eth_capabilities"}
+<< {"jsonrpc":"2.0","id":1,"result":{"head":{"blockNumber":"0x13f8e3a","blockHash":"0xd4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3"},"state":{"disabled":false,"oldestBlock":"0x1a2b3c","deleteStrategy":{"type":"window","retentionBlocks":50000}},"tx":{"disabled":false,"oldestBlock":"0x10203","deleteStrategy":{"type":"window","retentionBlocks":1200000}},"logs":{"disabled":false,"oldestBlock":"0x10203","deleteStrategy":{"type":"window","retentionBlocks":1200000}},"receipts":{"disabled":false,"oldestBlock":"0x10203","deleteStrategy":{"type":"window","retentionBlocks":1200000}},"blocks":{"disabled":false,"oldestBlock":"0x0","deleteStrategy":{"type":"none"}},"stateproofs":{"disabled":true,"oldestBlock":"0x0","deleteStrategy":{"type":"none"}}}}

tests/eth_capabilities/get-capabilities.io

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+// Returns effective routing capabilities.
+>> {"jsonrpc":"2.0","id":1,"method":"eth_capabilities"}
+<< {"jsonrpc":"2.0","id":1,"result":{"head":{"blockNumber":"0x13f8e3a","blockHash":"0xd4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3"},"state":{"disabled":false,"oldestBlock":"0x12ab34","deleteStrategy":{"type":"window","retentionBlocks":90000}},"tx":{"disabled":false,"oldestBlock":"0xf00aa","deleteStrategy":{"type":"window","retentionBlocks":2350000}},"logs":{"disabled":false,"oldestBlock":"0xf00aa","deleteStrategy":{"type":"window","retentionBlocks":2350000}},"receipts":{"disabled":false,"oldestBlock":"0xf00aa","deleteStrategy":{"type":"window","retentionBlocks":2350000}},"blocks":{"disabled":false,"oldestBlock":"0x0","deleteStrategy":{"type":"none"}},"stateproofs":{"disabled":false,"oldestBlock":"0x120000","deleteStrategy":{"type":"window","retentionBlocks":90000}}}}