refactor: 拆分 Rust gateway/executor 大文件为模块目录结构，拆分 Python gateway.py 为子模块

Rust 侧:
- executor.rs 拆分为 executor/ 目录 (plan_builders, stream, sync, submission)
- 新增 kiro_stream/, local_finalize/, local_stream/ 模块目录
- 新增 video_tasks.rs
- 测试文件 ai_execute/files/video 拆分为子目录
- handlers/headers/control/constants 扩展支持新模块

Python 侧:
- gateway.py 拆分为 24 个子模块 (routes, shared, contract, chat, cli, video, files, finalize, reporting 等)
- 新增 antigravity/gemini_cli/kiro 的 rust_http 适配层
- upstream_fetcher 增加 Rust sidecar 支持
- executor_plan/candidate/pipeline 适配调整

测试:
- 对应拆分 test_internal_gateway_routes 为子目录
- 新增 rust_http 相关测试

tests/services/test_gemini_cli_rust_http.py

@@ -0,0 +1,125 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+from unittest.mock import AsyncMock
+
+import httpx
+import pytest
+
+import src.services.provider.adapters.gemini_cli.rust_http as gemini_cli_rust_http_mod
+import src.services.request.rust_executor_client as rust_client_mod
+from src.services.provider.adapters.gemini_cli.client import (
+    load_code_assist,
+    onboard_user,
+)
+from src.services.request.rust_executor_client import (
+    RustExecutorClientError,
+    RustExecutorSyncResult,
+)
+
+
+@pytest.mark.asyncio
+async def test_load_code_assist_uses_rust_executor(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(gemini_cli_rust_http_mod.config, "executor_backend", "rust")
+    monkeypatch.setattr(
+        rust_client_mod.RustExecutorClient,
+        "execute_sync_json",
+        AsyncMock(
+            return_value=RustExecutorSyncResult(
+                status_code=200,
+                headers={"content-type": "application/json"},
+                response_json={"cloudaicompanionProject": "project-1"},
+            )
+        ),
+    )
+    monkeypatch.setattr(
+        "src.clients.http_client.HTTPClientPool.get_proxy_client",
+        AsyncMock(side_effect=AssertionError("python fallback should not run")),
+    )
+
+    result = await load_code_assist("access-token", proxy_config=None, timeout_seconds=12.0)
+
+    assert result["cloudaicompanionProject"] == "project-1"
+    plan = rust_client_mod.RustExecutorClient.execute_sync_json.await_args.args[0]
+    assert plan.method == "POST"
+    assert plan.url.endswith("/v1internal:loadCodeAssist")
+    assert plan.provider_api_format == "gemini_cli:load_code_assist"
+    assert plan.body.json_body == {
+        "metadata": {
+            "ideType": "ANTIGRAVITY",
+            "platform": "PLATFORM_UNSPECIFIED",
+            "pluginType": "GEMINI",
+        }
+    }
+
+
+@pytest.mark.asyncio
+async def test_load_code_assist_falls_back_to_python_when_rust_unavailable(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(gemini_cli_rust_http_mod.config, "executor_backend", "rust")
+    monkeypatch.setattr(
+        rust_client_mod.RustExecutorClient,
+        "execute_sync_json",
+        AsyncMock(side_effect=RustExecutorClientError("executor down")),
+    )
+    fake_client = SimpleNamespace(
+        post=AsyncMock(
+            return_value=httpx.Response(
+                status_code=200,
+                request=httpx.Request("POST", "https://example.invalid/v1internal:loadCodeAssist"),
+                json={"cloudaicompanionProject": "project-fallback"},
+            )
+        )
+    )
+    monkeypatch.setattr(
+        "src.clients.http_client.HTTPClientPool.get_proxy_client",
+        AsyncMock(return_value=fake_client),
+    )
+
+    result = await load_code_assist("access-token", proxy_config=None, timeout_seconds=12.0)
+
+    assert result["cloudaicompanionProject"] == "project-fallback"
+    fake_client.post.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_onboard_user_uses_rust_executor(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(gemini_cli_rust_http_mod.config, "executor_backend", "rust")
+    monkeypatch.setattr(
+        rust_client_mod.RustExecutorClient,
+        "execute_sync_json",
+        AsyncMock(
+            return_value=RustExecutorSyncResult(
+                status_code=200,
+                headers={"content-type": "application/json"},
+                response_json={
+                    "done": True,
+                    "response": {"cloudaicompanionProject": {"id": "project-onboarded"}},
+                },
+            )
+        ),
+    )
+    monkeypatch.setattr(
+        "src.clients.http_client.HTTPClientPool.get_proxy_client",
+        AsyncMock(side_effect=AssertionError("python fallback should not run")),
+    )
+
+    project_id = await onboard_user(
+        "access-token",
+        tier_id="legacy",
+        proxy_config=None,
+        timeout_seconds=15.0,
+        max_attempts=1,
+    )
+
+    assert project_id == "project-onboarded"
+    plan = rust_client_mod.RustExecutorClient.execute_sync_json.await_args.args[0]
+    assert plan.method == "POST"
+    assert plan.url.endswith("/v1internal:onboardUser")
+    assert plan.provider_api_format == "gemini_cli:onboard_user"
+    assert plan.body.json_body["tierId"] == "legacy"