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/api/internal_gateway/test_decision_plan_misc.py

@@ -0,0 +1,158 @@
+"""Gateway internal decision/plan builder tests."""
+
+import asyncio
+import base64
+import json
+import time
+from types import SimpleNamespace
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+from fastapi import BackgroundTasks, FastAPI
+from fastapi.responses import JSONResponse, StreamingResponse
+from fastapi.testclient import TestClient
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from src.api.internal.gateway import (
+    CONTROL_ACTION_HEADER,
+    CONTROL_ACTION_PROXY_PUBLIC,
+    CONTROL_EXECUTED_HEADER,
+    GatewayAuthContext,
+    GatewayExecutionDecisionResponse,
+    GatewayExecuteRequest,
+    GatewayExecutionPlanResponse,
+    GatewayResolveRequest,
+    GatewayStreamReportRequest,
+    GatewaySyncReportRequest,
+    _dispatch_gateway_sync_telemetry,
+    _build_gateway_sync_error_payload,
+    _build_gateway_sync_telemetry_writer,
+    _run_gateway_stream_report_background,
+    _run_gateway_sync_report_background,
+    _stream_executor_requires_python_rewrite,
+    _build_claude_chat_sync_decision,
+    _build_claude_chat_stream_decision,
+    _build_claude_cli_sync_decision,
+    _build_claude_cli_stream_decision,
+    _build_gemini_files_download_stream_decision,
+    _build_gemini_files_proxy_sync_decision,
+    _build_gemini_chat_sync_decision,
+    _build_gemini_chat_stream_decision,
+    _build_gemini_cli_sync_decision,
+    _build_gemini_cli_stream_decision,
+    _build_openai_chat_sync_decision,
+    _build_openai_chat_stream_decision,
+    _build_openai_cli_stream_decision,
+    _build_openai_video_content_stream_decision,
+    _extract_gateway_sync_error_message,
+    _record_gateway_direct_candidate_graph,
+    _resolve_gateway_sync_error_status_code,
+    _build_openai_chat_sync_plan,
+    _build_openai_cli_sync_decision,
+    _build_openai_cli_stream_plan,
+    _build_openai_cli_sync_plan,
+    _is_streaming_sync_payload,
+    _resolve_auth_context,
+    _resolve_gateway_sync_adapter,
+    classify_gateway_route,
+    router,
+)
+from src.database import get_db
+from src.models.database import Base, RequestCandidate
+from src.services.orchestration.candidate_resolver import CandidateResolver
+from src.services.scheduling.schemas import PoolCandidate, ProviderCandidate
+from src.services.request.executor_plan import ExecutionPlan, ExecutionPlanBody, PreparedExecutionPlan
+
+
+def _wait_until(predicate: Any, *, timeout: float = 1.0, interval: float = 0.01) -> None:
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        if predicate():
+            return
+        time.sleep(interval)
+    assert predicate()
+
+
+def test_stream_executor_requires_python_rewrite_allows_antigravity_same_format() -> None:
+    class AntigravityEnvelope:
+        name = "antigravity:v1internal"
+
+        def force_stream_rewrite(self) -> bool:
+            return True
+
+    envelope = AntigravityEnvelope()
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=False,
+            provider_api_format="gemini:cli",
+            client_api_format="gemini:cli",
+        )
+        is False
+    )
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=False,
+            provider_api_format="gemini:chat",
+            client_api_format="gemini:chat",
+        )
+        is False
+    )
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=False,
+            provider_api_format="gemini:cli",
+            client_api_format="openai:cli",
+        )
+        is True
+    )
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=True,
+            provider_api_format="gemini:chat",
+            client_api_format="openai:chat",
+        )
+        is False
+    )
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=True,
+            provider_api_format="gemini:cli",
+            client_api_format="openai:cli",
+        )
+        is False
+    )
+
+
+def test_stream_executor_requires_python_rewrite_allows_kiro_same_format() -> None:
+    class KiroEnvelope:
+        name = "kiro:generateAssistantResponse"
+
+        def force_stream_rewrite(self) -> bool:
+            return True
+
+    envelope = KiroEnvelope()
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=False,
+            provider_api_format="claude:cli",
+            client_api_format="claude:cli",
+        )
+        is False
+    )
+    assert (
+        _stream_executor_requires_python_rewrite(
+            envelope=envelope,
+            needs_conversion=False,
+            provider_api_format="claude:chat",
+            client_api_format="claude:chat",
+        )
+        is True
+    )