fix(models): propagate Anthropic finish reasons

[MukundaKatta]

Summary

• populate LlmResponse.finish_reason for non-streaming Anthropic responses using the existing stop-reason mapper
• capture message_delta.stop_reason during streaming and set it on the final aggregated response
• map Anthropic pause_turn to STOP and refusal to SAFETY, and add regression tests for the helper and both response paths

Testing

• python3 -m py_compile src/google/adk/models/anthropic_llm.py tests/unittests/models/test_anthropic_llm.py
• python3 -m pytest tests/unittests/models/test_anthropic_llm.py -k "finish_reason or non_streaming_does_not_pass_stream_param or streaming_sets_finish_reason_from_message_delta" (fails during collection locally: ModuleNotFoundError: No module named anthropic)

[adk-bot]

Response from ADK Triaging Agent

Hello @MukundaKatta, thank you for creating this PR!

This PR is a bug fix. Could you please associate a GitHub issue with this PR? If there is no existing issue, could you please create one?

This information will help reviewers to review your PR more efficiently. Thanks!