fix(provider): normalize bare gemini-3 Pro model IDs for google-antigravity (#24145)

* fix(provider): normalize bare gemini-3 Pro model IDs for google-antigravity

The Antigravity Cloud Code Assist API requires a thinking-tier suffix
(-low or -high) for all Gemini 3 Pro variants.  When a user configures
a bare model ID like `gemini-3.1-pro`, the API returns a 404 because it
only recognises `gemini-3.1-pro-low` or `gemini-3.1-pro-high`.

Add `normalizeAntigravityModelId()` that appends `-low` (the default
tier) to bare Pro model IDs, and apply it during provider normalisation
for `google-antigravity`.  Also refactor the per-provider model
normalisation into a shared `normalizeProviderModels()` helper.

Closes #24071

Co-authored-by: Cursor <cursoragent@cursor.com>

* Tests: cover antigravity model ID normalization

* Changelog: note antigravity pro tier normalization

* Tests: type antigravity model helper inputs

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/agents/models-config.providers.ts

@@ -391,10 +391,22 @@ export function normalizeGoogleModelId(id: string): string {
   return id;
 }
 
-function normalizeGoogleProvider(provider: ProviderConfig): ProviderConfig {
+const ANTIGRAVITY_BARE_PRO_IDS = new Set(["gemini-3-pro", "gemini-3.1-pro", "gemini-3-1-pro"]);
+
+export function normalizeAntigravityModelId(id: string): string {
+  if (ANTIGRAVITY_BARE_PRO_IDS.has(id)) {
+    return `${id}-low`;
+  }
+  return id;
+}
+
+function normalizeProviderModels(
+  provider: ProviderConfig,
+  normalizeId: (id: string) => string,
+): ProviderConfig {
   let mutated = false;
   const models = provider.models.map((model) => {
-    const nextId = normalizeGoogleModelId(model.id);
+    const nextId = normalizeId(model.id);
     if (nextId === model.id) {
       return model;
     }
@@ -404,6 +416,14 @@ function normalizeGoogleProvider(provider: ProviderConfig): ProviderConfig {
   return mutated ? { ...provider, models } : provider;
 }
 
+function normalizeGoogleProvider(provider: ProviderConfig): ProviderConfig {
+  return normalizeProviderModels(provider, normalizeGoogleModelId);
+}
+
+function normalizeAntigravityProvider(provider: ProviderConfig): ProviderConfig {
+  return normalizeProviderModels(provider, normalizeAntigravityModelId);
+}
+
 export function normalizeProviders(params: {
   providers: ModelsConfig["providers"];
   agentDir: string;
@@ -470,6 +490,14 @@ export function normalizeProviders(params: {
       normalizedProvider = googleNormalized;
     }
 
+    if (normalizedKey === "google-antigravity") {
+      const antigravityNormalized = normalizeAntigravityProvider(normalizedProvider);
+      if (antigravityNormalized !== normalizedProvider) {
+        mutated = true;
+      }
+      normalizedProvider = antigravityNormalized;
+    }
+
     next[key] = normalizedProvider;
   }