fix(mcp): correct Codex MCP configuration format to [mcp_servers]

BREAKING CHANGE: The [mcp.servers] format was completely incorrect and not
any official Codex format. The only correct format is [mcp_servers] at the
top level of config.toml.

Changes:
- Remove incorrect [mcp.servers] nested table support
- Always use [mcp_servers] top-level table (official Codex format)
- Auto-migrate and cleanup erroneous [mcp.servers] entries on write
- Preserve error-tolerant import for migrating old incorrect configs
- Simplify sync logic by removing format selection branches (~60 lines)
- Update all documentation and tests to reflect correct format
- Add warning logs when detecting and cleaning incorrect format

Backend (Rust):
- mcp.rs: Simplify sync_enabled_to_codex by removing Target enum
- mcp.rs: sync_single_server_to_codex now always uses [mcp_servers]
- mcp.rs: remove_server_from_codex cleans both locations
- mcp.rs: Update import_from_codex comments to clarify format status
- tests: Rename test to sync_enabled_to_codex_migrates_erroneous_*
- tests: Update assertions to verify migration behavior

Frontend (TypeScript):
- tomlUtils.ts: Prioritize [mcp_servers] format in parsing
- tomlUtils.ts: Update error messages to guide correct format

Documentation:
- README.md: Correct MCP format reference to [mcp_servers]
- CLAUDE.md: Add comprehensive format specification with examples

All 79 tests pass. This ensures backward compatibility while enforcing
the correct Codex official standard going forward.

Refs: https://github.com/openai/codex/issues/3441

src-tauri/tests/mcp_commands.rs

@@ -127,8 +127,14 @@ fn import_mcp_from_claude_creates_config_and_enables_servers() {
 
     let guard = state.config.read().expect("lock config");
     // v3.7.0: 检查统一结构
-    let servers = guard.mcp.servers.as_ref().expect("unified servers should exist");
-    let entry = servers.get("echo").expect("server imported into unified structure");
+    let servers = guard
+        .mcp
+        .servers
+        .as_ref()
+        .expect("unified servers should exist");
+    let entry = servers
+        .get("echo")
+        .expect("server imported into unified structure");
     assert!(
         entry.apps.claude,
         "imported server should have Claude app enabled"
@@ -182,10 +188,12 @@ fn set_mcp_enabled_for_codex_writes_live_config() {
     // 创建 Codex 配置目录和文件
     let codex_dir = home.join(".codex");
     fs::create_dir_all(&codex_dir).expect("create codex dir");
-    fs::write(codex_dir.join("auth.json"), r#"{"OPENAI_API_KEY":"test-key"}"#)
-        .expect("create auth.json");
-    fs::write(codex_dir.join("config.toml"), "")
-        .expect("create empty config.toml");
+    fs::write(
+        codex_dir.join("auth.json"),
+        r#"{"OPENAI_API_KEY":"test-key"}"#,
+    )
+    .expect("create auth.json");
+    fs::write(codex_dir.join("config.toml"), "").expect("create empty config.toml");
 
     let mut config = MultiAppConfig::default();
     config.ensure_app(&AppType::Codex);
@@ -203,7 +211,7 @@ fn set_mcp_enabled_for_codex_writes_live_config() {
             }),
             apps: McpApps {
                 claude: false,
-                codex: false,  // 初始未启用
+                codex: false, // 初始未启用
                 gemini: false,
             },
             description: None,