feat(Handler/workspace_fs_api): Implement core workspace FS API handlers

- Added `workspace_fs_api` handler module to `Mountain` backend for processing `workspace.fs` RPC methods required by VS Code extensions
- Implemented handlers for `stat`, `readFile`, `writeFile`, `readDirectory`, `createDirectory`, `delete`, `rename`, and `copy` operations
- Integrated with `FsReader`/`FsWriter` traits from environment to perform actual filesystem operations while enforcing scheme validation
- Added structured error mapping from `CommonError` to extension-friendly JSON-RPC errors with POSIX-style error codes (ENOENT, EACCES)
- Implemented URI scheme filtering to reject `vscode-webview`/`vscode-remote` URIs not supported by native FS operations
- Added base64 encoding/decoding for file content transfer to maintain binary safety over JSON-RPC

This implementation enables Cocoon extension host to provide filesystem API compatibility for VS Code extensions while maintaining security boundaries and proper error handling as required by the `fs-api-shim.js` layer.

Author: NikolaRHristov

Source/Library.rs

@@ -44,4 +44,6 @@ pub mod handlers {
 	pub mod ui;
 
 	pub mod workspace;
+
+	pub mod workspace_fs_api;
 }