fix(antigravity): preserve Claude tool delta index (#2223)

Gemini response translation wrote OpenAI-shaped bookkeeping into the
shared state.toolCalls map, which the downstream openai-to-claude
translator uses for Claude block metadata. That pre-population skipped
blockIndex creation, so Anthropic input_json_delta events lost index.

Track Gemini function calls via state.geminiToolCallCount instead,
leaving state.toolCalls clean for the Claude translator.

Closes #2218

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

tests/translator/bugs-antigravity.test.js

@@ -1,7 +1,7 @@
 // Real Antigravity-MITM requests (Gemini-internal: { request: { contents, ... } }) → OpenAI.
 import { describe, it, expect } from "vitest";
 import "./registerAll.js";
-import { translateRequest } from "../../open-sse/translator/index.js";
+import { translateRequest, translateResponse, initState } from "../../open-sse/translator/index.js";
 import { FORMATS } from "../../open-sse/translator/formats.js";
 import { AntigravityExecutor } from "../../open-sse/executors/antigravity.js";
 
@@ -54,6 +54,32 @@ describe("Antigravity → OpenAI", () => {
   });
 });
 
+describe("Antigravity → Claude", () => {
+  it("tool call input_json_delta includes Anthropic index", () => {
+    const state = initState(FORMATS.CLAUDE);
+    const events = translateResponse(FORMATS.ANTIGRAVITY, FORMATS.CLAUDE, {
+      response: {
+        responseId: "resp-1",
+        modelVersion: "gemini-pro-agent",
+        candidates: [{
+          content: {
+            role: "model",
+            parts: [{ functionCall: { name: "bash", args: { command: "git status" } } }],
+          },
+          finishReason: "STOP",
+          index: 0,
+        }],
+      },
+    }, state);
+
+    const jsonDelta = events.find(
+      (event) => event.type === "content_block_delta" && event.delta?.type === "input_json_delta"
+    );
+    expect(jsonDelta).toMatchObject({ index: expect.any(Number) });
+    expect(JSON.parse(jsonDelta.delta.partial_json)).toEqual({ command: "git status" });
+  });
+});
+
 describe("Antigravity executor", () => {
   it("strips optional from nested tool schemas", () => {
     const out = new AntigravityExecutor().transformRequest("gemini-2.5-pro", {

tests/translator/claude-kiro-direct.test.js

@@ -171,6 +171,7 @@ describe("Kiro → Claude (direct route, OpenAI-shaped chunks from executor)", (
     const jsonDelta = events.find(
       (e) => e.type === "content_block_delta" && e.delta.type === "input_json_delta"
     );
+    expect(jsonDelta.index).toBeDefined();
     expect(jsonDelta.delta.partial_json).toBe('{"q":"x"}');
     const md = events.find((e) => e.type === "message_delta");
     expect(md.delta.stop_reason).toBe("tool_use");