refactor(mcp): preserve TOML formatting when syncing to Codex

Switch from `toml` to `toml_edit` crate for incremental TOML editing,
preserving user-written comments, whitespace, and key ordering in
Codex config.toml.

Changes:
- Add toml_edit 0.22 dependency for preserving-style TOML editing
- Refactor sync_enabled_to_codex() to use toml_edit::DocumentMut API
- Implement smart style detection: inherit existing mcp.servers or
  mcp_servers style, with fallback to sensible defaults
- Add deduplication logic to prevent both styles coexisting
- Add tests for format preservation and style inheritance
- Fix unused_mut and nonminimal_bool compiler warnings
- Apply cargo fmt to all modified files

Benefits:
- User comments and formatting are no longer lost during sync
- Respects user's preferred TOML structure (nested vs toplevel)
- Non-MCP fields in config.toml remain untouched
- Minimal surprise principle: only modifies necessary sections

Testing:
- All 47 tests passing (unit + integration)
- Clippy clean (0 warnings, 0 errors)
- Release build successful
- New tests verify comment preservation and style detection

src-tauri/src/lib.rs

@@ -273,7 +273,7 @@ fn switch_provider_internal(
             Some(app_type_str.clone()),
             provider_id,
         )
-            .map_err(AppError::Message)?;
+        .map_err(AppError::Message)?;
 
         // 切换成功后重新创建托盘菜单
         if let Ok(new_menu) = create_tray_menu(app, app_state.inner()) {