app-server: add v2 filesystem APIs (#14245)

Add a protocol-level filesystem surface to the v2 app-server so Codex
clients can read and write files, inspect directories, and subscribe to
path changes without relying on host-specific helpers.

High-level changes:
- define the new v2 fs/readFile, fs/writeFile, fs/createDirectory,
fs/getMetadata, fs/readDirectory, fs/remove, fs/copy RPCs
- implement the app-server handlers, including absolute-path validation,
base64 file payloads, recursive copy/remove semantics
- document the API, regenerate protocol schemas/types, and add
end-to-end tests for filesystem operations, copy edge cases

Testing plan:
- validate protocol serialization and generated schema output for the
new fs request, response, and notification types
- run app-server integration coverage for file and directory CRUD paths,
metadata/readDirectory responses, copy failure modes, and absolute-path
validation

codex-rs/app-server/src/codex_message_processor.rs

@@ -899,6 +899,15 @@ impl CodexMessageProcessor {
             | ClientRequest::ConfigBatchWrite { .. } => {
                 warn!("Config request reached CodexMessageProcessor unexpectedly");
             }
+            ClientRequest::FsReadFile { .. }
+            | ClientRequest::FsWriteFile { .. }
+            | ClientRequest::FsCreateDirectory { .. }
+            | ClientRequest::FsGetMetadata { .. }
+            | ClientRequest::FsReadDirectory { .. }
+            | ClientRequest::FsRemove { .. }
+            | ClientRequest::FsCopy { .. } => {
+                warn!("Filesystem request reached CodexMessageProcessor unexpectedly");
+            }
             ClientRequest::ConfigRequirementsRead { .. } => {
                 warn!("ConfigRequirementsRead request reached CodexMessageProcessor unexpectedly");
             }