fix(models): wrap Anthropic rate limit errors

[MukundaKatta]

Summary

• wrap Anthropic in a dedicated ADK exception with mitigation guidance
• apply the wrapper consistently for both streaming and non-streaming Claude requests
• add regression tests for both code 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 "wraps_anthropic_rate_limit_error" (fails in this environment because the pytest interpreter is missing the package during collection)

[adk-bot]

Response from ADK Triaging Agent

Hello @MukundaKatta, thank you for your contribution!

To help us track and manage changes effectively, could you please associate this PR with a GitHub issue? If an issue doesn't exist yet, you can create one and link it.

This will help our reviewers better understand the context of your fix. Thanks!

[rohityan]

Hi @MukundaKatta , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Can you please fix the failing mypy-diff tests and formatting errors.