fix(agents): add forward-compat fallback for google-gemini-cli gemini-3.1-pro/flash-preview (#26570)

* fix(agents): add "google" provider to isReasoningTagProvider to prevent reasoning leak

The gemini-api-key auth flow creates a profile with provider "google"
(e.g. google/gemini-3-pro-preview), but isReasoningTagProvider only
matched "google-gemini-cli" (OAuth) and "google-generative-ai". As a
result:
- reasoningTagHint was false → system prompt omitted <think>/<final>
  formatting instructions
- enforceFinalTag was false → <final> tag filtering was skipped

Raw <think> reasoning output was delivered to the end user.

Fix: add the bare "google" provider string to the match list and cover
it with two new test cases (exact match + case-insensitive).

Fixes #26551

* fix(agents): add forward-compat fallback for google-gemini-cli gemini-3.1-pro/flash-preview

gemini-3.1-pro-preview and gemini-3.1-flash-preview are not yet present in
pi-ai's built-in google-gemini-cli model catalog (only gemini-3-pro-preview
and gemini-3-flash-preview are registered). When users configure these models
they get "Unknown model" errors even though Gemini CLI OAuth supports them.

The codebase already has isGemini31Model() in extra-params.ts, which proves
intent to support these models. Add a resolveGoogleGeminiCli31ForwardCompatModel
entry to resolveForwardCompatModel following the same clone-template pattern
used for zai/glm-5 and anthropic 4.6 models.

- gemini-3.1-pro-* clones gemini-3-pro-preview (with reasoning: true)
- gemini-3.1-flash-* clones gemini-3-flash-preview (with reasoning: true)

Also add test helpers and three test cases to model.forward-compat.test.ts.

Fixes #26524

* Changelog: credit Google Gemini provider fallback fixes

---------

Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/agents/model-forward-compat.ts

@@ -17,6 +17,14 @@ const ANTHROPIC_SONNET_TEMPLATE_MODEL_IDS = ["claude-sonnet-4-5", "claude-sonnet
 const ZAI_GLM5_MODEL_ID = "glm-5";
 const ZAI_GLM5_TEMPLATE_MODEL_IDS = ["glm-4.7"] as const;
 
+// gemini-3.1-pro-preview / gemini-3.1-flash-preview are not yet in pi-ai's built-in
+// google-gemini-cli catalog. Clone the gemini-3-pro/flash-preview template so users
+// don't get "Unknown model" errors when Google releases a new minor version.
+const GEMINI_3_1_PRO_PREFIX = "gemini-3.1-pro";
+const GEMINI_3_1_FLASH_PREFIX = "gemini-3.1-flash";
+const GEMINI_3_1_PRO_TEMPLATE_IDS = ["gemini-3-pro-preview"] as const;
+const GEMINI_3_1_FLASH_TEMPLATE_IDS = ["gemini-3-flash-preview"] as const;
+
 function cloneFirstTemplateModel(params: {
   normalizedProvider: string;
   trimmedModelId: string;
@@ -160,6 +168,38 @@ function resolveAnthropicSonnet46ForwardCompatModel(
   });
 }
 
+// gemini-3.1-pro-preview / gemini-3.1-flash-preview are not present in pi-ai's built-in
+// google-gemini-cli catalog yet. Clone the nearest gemini-3 template so users don't get
+// "Unknown model" errors when Google Gemini CLI gains new minor-version models.
+function resolveGoogleGeminiCli31ForwardCompatModel(
+  provider: string,
+  modelId: string,
+  modelRegistry: ModelRegistry,
+): Model<Api> | undefined {
+  if (normalizeProviderId(provider) !== "google-gemini-cli") {
+    return undefined;
+  }
+  const trimmed = modelId.trim();
+  const lower = trimmed.toLowerCase();
+
+  let templateIds: readonly string[];
+  if (lower.startsWith(GEMINI_3_1_PRO_PREFIX)) {
+    templateIds = GEMINI_3_1_PRO_TEMPLATE_IDS;
+  } else if (lower.startsWith(GEMINI_3_1_FLASH_PREFIX)) {
+    templateIds = GEMINI_3_1_FLASH_TEMPLATE_IDS;
+  } else {
+    return undefined;
+  }
+
+  return cloneFirstTemplateModel({
+    normalizedProvider: "google-gemini-cli",
+    trimmedModelId: trimmed,
+    templateIds: [...templateIds],
+    modelRegistry,
+    patch: { reasoning: true },
+  });
+}
+
 // Z.ai's GLM-5 may not be present in pi-ai's built-in model catalog yet.
 // When a user configures zai/glm-5 without a models.json entry, clone glm-4.7 as a forward-compat fallback.
 function resolveZaiGlm5ForwardCompatModel(
@@ -211,6 +251,7 @@ export function resolveForwardCompatModel(
     resolveOpenAICodexGpt53FallbackModel(provider, modelId, modelRegistry) ??
     resolveAnthropicOpus46ForwardCompatModel(provider, modelId, modelRegistry) ??
     resolveAnthropicSonnet46ForwardCompatModel(provider, modelId, modelRegistry) ??
-    resolveZaiGlm5ForwardCompatModel(provider, modelId, modelRegistry)
+    resolveZaiGlm5ForwardCompatModel(provider, modelId, modelRegistry) ??
+    resolveGoogleGeminiCli31ForwardCompatModel(provider, modelId, modelRegistry)
   );
 }