fix: preserve Anthropic model version in streaming and non-streaming responses

[aaravxsingh15]

Description

Populates LlmResponse.model_version for Anthropic responses using the resolved
model returned by the Anthropic API.

• Set model_version from message.model for non-streaming responses.
• Capture the resolved model from message_start for streaming responses.
• Include the resolved model in the final aggregated streaming response.
• Add regression tests covering both response paths.

Fixes #6847.

Testing plan

• python -m pytest tests/unittests/models/test_anthropic_llm.py -q
• python -m pytest tests/unittests/models/test_anthropic_llm.py -k "model_version" -q
• python -m ruff check src/google/adk/models/anthropic_llm.py

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