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/hooks/runtime-fallback/constants.ts

@@ -31,6 +31,8 @@ export const RETRYABLE_ERROR_PATTERNS = [
   /cool(?:ing)?\s+down/i,
   /exhausted\s+your\s+capacity/i,
   /usage\s+limit\s+has\s+been\s+reached/i,
+  /model.{0,20}?not.{0,10}?supported/i,
+  /model_not_supported/i,
   /service.?unavailable/i,
   /overloaded/i,
   /temporarily.?unavailable/i,

src/hooks/runtime-fallback/error-classifier.test.ts

@@ -276,4 +276,21 @@ describe("quota error detection (fixes #2747)", () => {
     expect(errorType).toBe("quota_exceeded")
     expect(retryable).toBe(true)
   })
+
+  test("detects model_not_supported errors as retryable for fallback chain", () => {
+    //#given
+    const error1 = { message: "model_not_supported" }
+    const error2 = { message: "The model 'gpt-4-foo' is not supported by this API" }
+    const error3 = { message: "model not supported on free tier" }
+
+    //#when
+    const retryable1 = isRetryableError(error1, [400, 404])
+    const retryable2 = isRetryableError(error2, [400, 404])
+    const retryable3 = isRetryableError(error3, [400, 404])
+
+    //#then
+    expect(retryable1).toBe(true)
+    expect(retryable2).toBe(true)
+    expect(retryable3).toBe(true)
+  })
 })

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",