fix(ai): preserve unsigned tool call context for Gemini 3 with anti-mimicry note

Instead of skipping unsigned tool calls entirely (which lobotomizes context),
convert them to text with an explicit note telling the model this is historical
context from a different model and not a format to mimic.

This preserves tool call/result context when switching from providers without
thought signatures (e.g. Claude via Antigravity) to Gemini 3.

packages/ai/test/google-shared-gemini3-unsigned-tool-call.test.ts

@@ -63,7 +63,10 @@ describe("google-shared convertMessages", () => {
 		expect(toolTurn?.parts?.some((p) => p.functionCall !== undefined)).toBe(false);
 
 		const text = toolTurn?.parts?.map((p) => p.text ?? "").join("\n");
-		expect(text).toContain("[Tool Call: bash]");
+		// Should contain historical context note to prevent mimicry
+		expect(text).toContain("Historical context");
+		expect(text).toContain("bash");
 		expect(text).toContain("ls -la");
+		expect(text).toContain("Do not mimic this format");
 	});
 });