feat(ai/providers): added forced tool directive for antigravity gemini routes

- Added a markdown template defining a tool-only turn directive for Gemini models.
- Appended the forced tool directive to request contents when using Antigravity Gemini routes with forced tool mode.

packages/ai/src/providers/google-antigravity-forced-tool.md

@@ -0,0 +1 @@
+TOOL-ONLY TURN. This turn accepts a tool call and nothing else; a text reply here is discarded unread and you will be re-prompted. Emit the tool call now.

packages/ai/src/providers/google-gemini-cli.ts

@@ -14,6 +14,7 @@ import {
 	getGeminiCliHeaders,
 } from "@oh-my-pi/pi-catalog/wire/gemini-headers";
 import { extractHttpStatusFromError, fetchWithRetry, readSseJson } from "@oh-my-pi/pi-utils";
+import forcedToolDirective from "./google-antigravity-forced-tool.md" with { type: "text" };
 import * as AIError from "../error";
 import type {
 	Api,
@@ -1347,6 +1348,13 @@ export function buildRequest(
 					},
 				};
 			}
+			// Cloud Code Assist drops `toolConfig` on Antigravity's Gemini routes:
+			// the backend answers in text under `mode: "ANY"` and still emits calls
+			// under `"NONE"`. Claude routes implement it, so only Gemini needs the
+			// forced choice restated in the transcript.
+			if (isAntigravity && !isClaudeModel(model.id) && request.toolConfig?.functionCallingConfig.mode === "ANY") {
+				contents.push({ role: "user", parts: [{ text: forcedToolDirective }] });
+			}
 		}
 		// Antigravity's default tool mode is VALIDATED (verified for Gemini and
 		// Claude); an explicit non-auto tool choice above wins.