feat(model): Add FallbackModel with circuit breaker

[mbutler-aws]

Description

This PR implements FallbackModel, a new model wrapper that provides automatic failover between any two model instances with circuit breaker protection.

Related Issues

N/A

Documentation PR

strands-agents/docs#369

Type of Change

New feature

Testing

Yes. PR includes both unit tests (29) with 91% code coverage, and integration tests (7):

Unit Tests

• Fallback triggering on throttle and connection errors
• Circuit breaker behavior (opening, closing, cooldown)
• Configuration and statistics tracking
• Streaming and structured output with fallback
• Error classification and re-raising

Integration Tests

• Same-provider fallback (Bedrock→Bedrock)

• Cross-provider fallback (OpenAI→Bedrock, Anthropic→Bedrock)

• Agent integration with FallbackModel

• Tool calling with real models

• Statistics tracking with real API calls

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[dnzprmksz]

Hey @mbutler-aws I was looking forward for this feature. Is it abandoned or will there be future work?

[mbutler-aws]

Added supporting documentation PR (strands-agents/docs#369). Unit tests and integration tests passed locally. Ready for review.

[mbutler-aws]

Hey @mbutler-aws I was looking forward for this feature. Is it abandoned or will there be future work?

Thanks for the nudge @dnzprmksz. Created the documentation PR today for the functionality. Hopefully can get this PR integrated.