refactor(backend): phase 4 - add test hooks and extend service layer

- Extract internal functions in commands/mcp.rs and commands/provider.rs
  to enable unit testing without Tauri context
- Add test hooks: set_mcp_enabled_test_hook, import_mcp_from_claude_test_hook,
  import_mcp_from_codex_test_hook, import_default_config_test_hook
- Migrate error types from String to AppError for precise error matching in tests
- Extend ProviderService with delete() method to unify Codex/Claude cleanup logic
- Add comprehensive test coverage:
  - tests/mcp_commands.rs: command-level tests for MCP operations
  - tests/provider_service.rs: service-level tests for switch/delete operations
- Run cargo fmt to fix formatting issues (EOF newlines)
- Update BACKEND_REFACTOR_PLAN.md to mark phase 3 complete

src-tauri/src/claude_plugin.rs

@@ -11,8 +11,7 @@ fn claude_dir() -> Result<PathBuf, AppError> {
     if let Some(dir) = crate::settings::get_claude_override_dir() {
         return Ok(dir);
     }
-    let home = dirs::home_dir()
-        .ok_or_else(|| AppError::Config("无法获取用户主目录".into()))?;
+    let home = dirs::home_dir().ok_or_else(|| AppError::Config("无法获取用户主目录".into()))?;
     Ok(home.join(CLAUDE_DIR))
 }
 
@@ -81,8 +80,7 @@ pub fn write_claude_config() -> Result<bool, AppError> {
     if changed || !path.exists() {
         let serialized = serde_json::to_string_pretty(&obj)
             .map_err(|e| AppError::JsonSerialize { source: e })?;
-        fs::write(&path, format!("{}\n", serialized))
-            .map_err(|e| AppError::io(&path, e))?;
+        fs::write(&path, format!("{}\n", serialized)).map_err(|e| AppError::io(&path, e))?;
         Ok(true)
     } else {
         Ok(false)
@@ -114,8 +112,8 @@ pub fn clear_claude_config() -> Result<bool, AppError> {
         return Ok(false);
     }
 
-    let serialized = serde_json::to_string_pretty(&value)
-        .map_err(|e| AppError::JsonSerialize { source: e })?;
+    let serialized =
+        serde_json::to_string_pretty(&value).map_err(|e| AppError::JsonSerialize { source: e })?;
     fs::write(&path, format!("{}\n", serialized)).map_err(|e| AppError::io(&path, e))?;
     Ok(true)
 }