fix(#2885): add model_not_supported to RETRYABLE error patterns

model_not_supported errors from providers (e.g. OpenAI returning
{"error": {"code": "model_not_supported"}}) were not recognized as
retryable. Subagents would silently fail with no response, hanging the
parent session.

Fix:
- Add "model_not_supported", "model not supported", "model is not
  supported" to RETRYABLE_MESSAGE_PATTERNS in model-error-classifier.ts
- Add regex patterns to RETRYABLE_ERROR_PATTERNS in
  runtime-fallback/constants.ts to match "model ... is ... not ...
  supported" with flexible spacing
- Add regression test covering all three variations

Now model_not_supported errors trigger the normal fallback chain instead
of silent failure.

src/shared/model-error-classifier.ts

@@ -51,6 +51,9 @@ const RETRYABLE_MESSAGE_PATTERNS = [
   "bad gateway",
   "unknown provider",
   "provider not found",
+  "model_not_supported",
+  "model not supported",
+  "model is not supported",
   "connection error",
   "network error",
   "timeout",