fix(anthropic): disable vendor SDK retries by default

[axelray-dev]

Summary

Fixes #6603.

livekit-plugins-anthropic was building anthropic.AsyncClient without max_retries, so the SDK default of 2 internal retries stacked on top of framework conn_options.max_retry. That made wall-clock time exceed conn_options.timeout (especially with FallbackAdapter).

OpenAI plugin already defaults vendor client retries to 0. This PR matches that for Anthropic.

Changes

• Pass max_retries=0 by default when constructing anthropic.AsyncClient in the Anthropic LLM plugin.
• Optional max_retries constructor arg for callers who want a different vendor-client policy.
• Unit test that the default client has max_retries == 0.
• Custom client= injection is unchanged (caller owns that client).

Verification

• ruff format / ruff check on touched files
• pytest tests/test_plugin_anthropic.py (8 passed)

Backwards compatibility

Default behavior for the constructed client changes from SDK default (2) to 0 so retries are owned by the framework only. Callers who pass their own client= are unaffected. Callers who want SDK retries can pass max_retries=....

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