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/pi-embedded-runner/model.forward-compat.test.ts

@@ -8,7 +8,11 @@ vi.mock("../pi-model-discovery.js", () => ({
 import { buildInlineProviderModels, resolveModel } from "./model.js";
 import {
   buildOpenAICodexForwardCompatExpectation,
+  GOOGLE_GEMINI_CLI_FLASH_TEMPLATE_MODEL,
+  GOOGLE_GEMINI_CLI_PRO_TEMPLATE_MODEL,
   makeModel,
+  mockGoogleGeminiCliFlashTemplateModel,
+  mockGoogleGeminiCliProTemplateModel,
   mockOpenAICodexTemplateModel,
   resetMockDiscoverModels,
 } from "./model.test-harness.js";
@@ -50,4 +54,36 @@ describe("pi embedded model e2e smoke", () => {
     expect(result.model).toBeUndefined();
     expect(result.error).toBe("Unknown model: openai-codex/gpt-4.1-mini");
   });
+
+  it("builds a google-gemini-cli forward-compat fallback for gemini-3.1-pro-preview", () => {
+    mockGoogleGeminiCliProTemplateModel();
+
+    const result = resolveModel("google-gemini-cli", "gemini-3.1-pro-preview", "/tmp/agent");
+    expect(result.error).toBeUndefined();
+    expect(result.model).toMatchObject({
+      ...GOOGLE_GEMINI_CLI_PRO_TEMPLATE_MODEL,
+      id: "gemini-3.1-pro-preview",
+      name: "gemini-3.1-pro-preview",
+      reasoning: true,
+    });
+  });
+
+  it("builds a google-gemini-cli forward-compat fallback for gemini-3.1-flash-preview", () => {
+    mockGoogleGeminiCliFlashTemplateModel();
+
+    const result = resolveModel("google-gemini-cli", "gemini-3.1-flash-preview", "/tmp/agent");
+    expect(result.error).toBeUndefined();
+    expect(result.model).toMatchObject({
+      ...GOOGLE_GEMINI_CLI_FLASH_TEMPLATE_MODEL,
+      id: "gemini-3.1-flash-preview",
+      name: "gemini-3.1-flash-preview",
+      reasoning: true,
+    });
+  });
+
+  it("keeps unknown-model errors for unrecognized google-gemini-cli model IDs", () => {
+    const result = resolveModel("google-gemini-cli", "gemini-4-unknown", "/tmp/agent");
+    expect(result.model).toBeUndefined();
+    expect(result.error).toBe("Unknown model: google-gemini-cli/gemini-4-unknown");
+  });
 });

src/agents/pi-embedded-runner/model.test-harness.ts

@@ -47,6 +47,48 @@ export function buildOpenAICodexForwardCompatExpectation(
   };
 }
 
+export const GOOGLE_GEMINI_CLI_PRO_TEMPLATE_MODEL = {
+  id: "gemini-3-pro-preview",
+  name: "Gemini 3 Pro Preview (Cloud Code Assist)",
+  provider: "google-gemini-cli",
+  api: "google-gemini-cli",
+  baseUrl: "https://cloudcode-pa.googleapis.com",
+  reasoning: true,
+  input: ["text", "image"] as const,
+  cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+  contextWindow: 200000,
+  maxTokens: 64000,
+};
+
+export const GOOGLE_GEMINI_CLI_FLASH_TEMPLATE_MODEL = {
+  id: "gemini-3-flash-preview",
+  name: "Gemini 3 Flash Preview (Cloud Code Assist)",
+  provider: "google-gemini-cli",
+  api: "google-gemini-cli",
+  baseUrl: "https://cloudcode-pa.googleapis.com",
+  reasoning: false,
+  input: ["text", "image"] as const,
+  cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+  contextWindow: 200000,
+  maxTokens: 64000,
+};
+
+export function mockGoogleGeminiCliProTemplateModel(): void {
+  mockDiscoveredModel({
+    provider: "google-gemini-cli",
+    modelId: "gemini-3-pro-preview",
+    templateModel: GOOGLE_GEMINI_CLI_PRO_TEMPLATE_MODEL,
+  });
+}
+
+export function mockGoogleGeminiCliFlashTemplateModel(): void {
+  mockDiscoveredModel({
+    provider: "google-gemini-cli",
+    modelId: "gemini-3-flash-preview",
+    templateModel: GOOGLE_GEMINI_CLI_FLASH_TEMPLATE_MODEL,
+  });
+}
+
 export function resetMockDiscoverModels(): void {
   vi.mocked(discoverModels).mockReturnValue({
     find: vi.fn(() => null),