fix(models): populate finish_reason on AnthropicLlm responses

[sebastienc]

Summary

Fixes #5394

Disclaimer: I'm using Claude Code to generate the following text. I find the summary is better than what I would've wrote.

AnthropicLlm was calling to_google_genai_finish_reason() but never wiring its result into LlmResponse, so finish_reason was always None. This fix connects it end-to-end and expands the mapping to cover all stop reasons the current Anthropic API can return.

• to_google_genai_finish_reason — extended to cover pause_turn → STOP (streaming interruption, e.g. server-side tool use) and refusal → SAFETY (model refusal); all other unknown values continue to return FINISH_REASON_UNSPECIFIED
• message_to_generate_content_response — passes stop_reason through to_google_genai_finish_reason and sets it on the returned LlmResponse
• _generate_content_streaming — reads stop_reason from the message_delta event and sets finish_reason on the final aggregated LlmResponse

Testing plan

10 new tests added to tests/unittests/models/test_anthropic_llm.py. Full suite: 69 passed.

tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[end_turn-STOP] PASSED
tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[stop_sequence-STOP] PASSED
tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[tool_use-STOP] PASSED
tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[max_tokens-MAX_TOKENS] PASSED
tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[pause_turn-STOP] PASSED
tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[refusal-SAFETY] PASSED
tests/unittests/models/test_anthropic_llm.py::test_to_google_genai_finish_reason[None-FINISH_REASON_UNSPECIFIED] PASSED
tests/unittests/models/test_anthropic_llm.py::test_non_streaming_sets_finish_reason PASSED
tests/unittests/models/test_anthropic_llm.py::test_non_streaming_finish_reason_max_tokens PASSED
tests/unittests/models/test_anthropic_llm.py::test_streaming_sets_finish_reason PASSED