fix(providers): add "field required" to failover error patterns

Add "field required" to ERROR_PATTERNS.format array to trigger auth
profile rotation and model failover when Google Antigravity returns
validation errors.

Fixes #3777

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agents/pi-embedded-helpers.iscloudcodeassistformaterror.test.ts

@@ -16,6 +16,7 @@ describe("isCloudCodeAssistFormatError", () => {
       "messages.1.content.1.tool_use.id",
       "tool_use.id should match pattern",
       "invalid request format",
+      "Field required",
     ];
     for (const sample of samples) {
       expect(isCloudCodeAssistFormatError(sample)).toBe(true);

src/agents/pi-embedded-helpers/errors.ts

@@ -395,6 +395,7 @@ const ERROR_PATTERNS = {
     "tool_use_id",
     "messages.1.content.1.tool_use.id",
     "invalid request format",
+    "field required",
   ],
 } as const;