Function withModelNotFoundFallback

Decorate an IModelAdapter with retire-and-retry. Behaviour:

1. Forward complete(request) to the inner adapter.
2. On MODEL_NOT_FOUND: refresh the cache, find the closest same-family alternative, retry once with request.model = <fallback>. Original error is returned if no fallback found.
3. The wrapper returns the SECOND error if the retry also fails.
4. stream() retries via restart-from-zero (#2550): when the inner stream throws a MODEL_NOT_FOUND ModelError, the wrapper closes the failed stream, picks a same-family fallback via the cache + registry, builds a new adapter through adapterFactory, and yields chunks from the fallback stream. The consumer sees a clean second stream — partial content already delivered by the first stream is NOT replayed; that's a known trade-off (the alternative resume-with-reconciliation strategy is heavier and only useful for tool-use loops; not implemented per the deliberate scoping in #2550). Without adapterFactory, the original throw propagates unchanged.
5. countTokens, validateConfig, listModels are passthrough.