[Feat][Router]: add fallback model support

[max-wittig]

When all backends for a model are unavailable (either health-checked away or all attempts errored out), requests automatically fall through to a configured fallback model. The model name in the request body is rewritten so downstream gateways (e.g. Envoy AI Gateway routing to Bedrock) receive the correct model identifier.

Config: per-model fallback_model in YAML, or --static-fallback-models CLI flag.

---

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[gemini-code-assist]

Code Review

This pull request implements fallback model support, enabling the router to redirect requests to an alternative model when primary backends are unhealthy or fail after multiple attempts. The implementation includes updates to documentation, configuration parsing for CLI and YAML, and the core routing logic. Review feedback highlights opportunities to improve the request rewriting process by updating the request body in-place to avoid synchronization issues with stale data. Additionally, it is recommended to refactor the fallback execution logic to reduce code duplication and ensure that fallback models utilize the same robust retry mechanisms as primary models.