feat(usage): add test script API with refactored execution logic

- Add private helper method `execute_and_format_usage_result` to eliminate code duplication
- Refactor `query_usage` to use helper method instead of duplicating result processing
- Add new `test_usage_script` method to test temporary script without saving
- Add backend command `test_usage_script` accepting script content as parameter
- Register new command in lib.rs invoke_handler
- Add frontend `usageApi.testScript` method to call the new backend API
- Update `UsageScriptModal.handleTest` to test current editor content instead of saved script
- Improve DX: users can now test script changes before saving

src-tauri/src/lib.rs

@@ -540,6 +540,7 @@ pub fn run() {
             commands::validate_mcp_command,
             // usage query
             commands::query_provider_usage,
+            commands::test_usage_script,
             // New MCP via config.json (SSOT)
             commands::get_mcp_config,
             commands::upsert_mcp_server_in_config,