Translate LLM backend capacity failures

internal/framework/contracts/errors.go

@@ -5,3 +5,7 @@ import "errors"
 // ErrInvalidStructuredOutput identifies a provider response that cannot satisfy
 // the caller's declared structured-output contract.
 var ErrInvalidStructuredOutput = errors.New("invalid structured output")
+
+// ErrLLMCapacityExceeded identifies backend admission exhaustion before model
+// generation begins.
+var ErrLLMCapacityExceeded = errors.New("LLM capacity exceeded")