Fix Anthropic streaming to convert SSE chunks

[matdev83]

Summary

• convert OpenAI-formatted streaming chunks to Anthropic SSE events inside AnthropicController
• add a focused unit test that exercises the streaming conversion end-to-end

Testing

• python -m pytest -o addopts='' tests/unit/anthropic_frontend_tests/test_anthropic_controller_streaming.py
• python -m pytest -o addopts='' tests/unit/anthropic_frontend_tests
• python -m pytest -o addopts='' (fails: missing optional test dependencies such as pytest-asyncio, respx, pytest_httpx)

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https://chatgpt.com/codex/tasks/task_e_68e6e2c736808333b42e5db683438675

[matdev83]

✅ APPROVED - This PR properly fixes Anthropic streaming by converting OpenAI-formatted SSE chunks to Anthropic format inside the AnthropicController.

Review Summary:

• ✅ Base branch is correctly set to 'dev'
• ✅ Code changes are focused and well-implemented
• ✅ Added proper streaming conversion with async iterator handling
• ✅ Includes comprehensive test coverage for the streaming conversion
• ✅ All related tests pass (55/55 anthropic frontend tests)
• ✅ Test failures in full suite are pre-existing issues not related to this PR

Technical Details:

• Properly handles both async and sync iterators in chunk conversion
• Maintains SSE headers for Anthropic compatibility
• Clean error handling with graceful fallbacks
• Test validates end-to-end streaming conversion

Safe to merge.