fix(beta): strip betas from per-request batch params

[Ashutosh0x]

fix(beta): strip betas from per-request batch params

Summary

The Message Batches API only supports beta flags at the top level (sent as a header for the batch create request). Including them in the JSON body for individual requests inside the batch causes API errors like output_format: Extra inputs are not permitted.

This PR ensures that betas are stripped from individual request parameters in both synchronous and asynchronous batch creation.

It also adds the missing "structured-outputs-2025-11-13" string to the AnthropicBetaParam literal to provide correct type hints for structured outputs.

Problem

Fixes #1118

When a user provides betas inside a batch request:

client.beta.messages.batches.create(
    requests=[
        Request(
            params=MessageCreateParamsNonStreaming(
                # ...
                betas=["structured-outputs-2025-11-13"]
            )
        )
    ]
)

The SDK was including "anthropic-beta": ["structured-outputs-2025-11-13"] in the serialized JSON body for that specific request, which the API does not allow for batch members.

Changes

• Updated src/anthropic/types/anthropic_beta_param.py to include "structured-outputs-2025-11-13".
• Updated src/anthropic/resources/beta/messages/batches.py to strip the betas key from each request's params before transformation and submission.

Verification

Verified with a reproduction script that mocks the internal _post method and checks the transmitted body. The anthropic-beta field is now correctly removed from individual batch requests.