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

Cargo.toml

@@ -30,6 +30,7 @@ tokio = { version = "1", features = ["macros", "net", "rt-multi-thread", "sync",
 tokio-util = { version = "0.7", features = ["codec", "io-util"] }
 uuid = { version = "1", features = ["serde", "v4"] }
 webpki-roots = "0.26"
+url = "2"
 
 [profile.release]
 lto = true