fix: set provider_id and model_id on FauxProvider streamed messages

[xfgong]

Summary

• FauxProvider now sets provider_id and model_id on streamed AssistantMessage objects, matching the behavior of real providers (e.g., Anthropic).
• The metadata is propagated both to partial events during streaming and to the final result message.
• Two new tests verify the fix on both the final result and partial stream events.

Closes #37

Test plan

• test_provider_metadata_set verifies provider_id and model_id on the final streamed result
• test_provider_metadata_on_partial_events verifies the fields on the start event's partial message
• Full test suite passes (289 tests, 0 failures)
• Ruff check and format pass