feat(rig-core): add ToolCallContext for per-call runtime context in tool dispatch

[redroy44]

Summary

• Adds ToolCallContext, a type-erased, cloneable type-map (modeled after http::Extensions' AnyClone pattern) that lets callers inject per-call runtime values (auth tokens, session IDs, conversation state) into tool execution
• Threads context through the full dispatch chain: ToolServerHandle → channel → ToolServer → ToolSet → ToolType → ToolDyn → Tool
• Fully backward-compatible: all existing Tool and ToolDyn implementations continue to work without changes

New public API

Item	Description
ToolCallContext (struct)	Type-map with new(), insert<T>(), get<T>(), get_mut<T>(), remove<T>(), contains<T>(). Re-exported from crate root.
Tool::call_with_context(args, &ctx)	Default method — delegates to call(). Override to access context.
ToolDyn::call_with_context(args, ctx)	Default method — delegates to call(). Blanket impl threads to Tool::call_with_context.
ToolSet::call_with_context(name, args, ctx)	Dispatches to tool with context.
ToolServerHandle::call_tool_with_context(name, args, ctx)	Sends context across the channel to the tool server.

No existing public API signatures were changed. No breaking changes.

Design

ToolCallContext uses Option<Box<HashMap<TypeId, Box<dyn AnyClone>>>> so empty contexts (the common path) have zero allocation. WASM-compatible via cfg(target_family = "wasm") gating on Send + Sync bounds.

Delegation direction:

• Dispatch layers (handle → set → dyn): old method delegates to new _with_context method + empty context
• Tool trait: new call_with_context default delegates to call (ignoring context)

Test plan

• 12 unit tests for ToolCallContext (insert/get/get_mut/remove/clone/contains/debug/empty allocation)
• 3 integration tests proving context flows end-to-end through ToolServer dispatch
• All 454 pre-existing + new lib tests pass
• cargo clippy --lib --tests -- -D warnings clean
• cargo fmt clean

This PR was generated with AI assistance (Claude).

Fixes #1536