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-protocol/src/protocol/common.rs

@@ -312,6 +312,34 @@ client_request_definitions! {
         params: v2::AppsListParams,
         response: v2::AppsListResponse,
     },
+    FsReadFile => "fs/readFile" {
+        params: v2::FsReadFileParams,
+        response: v2::FsReadFileResponse,
+    },
+    FsWriteFile => "fs/writeFile" {
+        params: v2::FsWriteFileParams,
+        response: v2::FsWriteFileResponse,
+    },
+    FsCreateDirectory => "fs/createDirectory" {
+        params: v2::FsCreateDirectoryParams,
+        response: v2::FsCreateDirectoryResponse,
+    },
+    FsGetMetadata => "fs/getMetadata" {
+        params: v2::FsGetMetadataParams,
+        response: v2::FsGetMetadataResponse,
+    },
+    FsReadDirectory => "fs/readDirectory" {
+        params: v2::FsReadDirectoryParams,
+        response: v2::FsReadDirectoryResponse,
+    },
+    FsRemove => "fs/remove" {
+        params: v2::FsRemoveParams,
+        response: v2::FsRemoveResponse,
+    },
+    FsCopy => "fs/copy" {
+        params: v2::FsCopyParams,
+        response: v2::FsCopyResponse,
+    },
     SkillsConfigWrite => "skills/config/write" {
         params: v2::SkillsConfigWriteParams,
         response: v2::SkillsConfigWriteResponse,
@@ -921,8 +949,17 @@ mod tests {
     use serde_json::json;
     use std::path::PathBuf;
 
+    fn absolute_path_string(path: &str) -> String {
+        let trimmed = path.trim_start_matches('/');
+        if cfg!(windows) {
+            format!(r"C:\{}", trimmed.replace('/', "\\"))
+        } else {
+            format!("/{trimmed}")
+        }
+    }
+
     fn absolute_path(path: &str) -> AbsolutePathBuf {
-        AbsolutePathBuf::from_absolute_path(path).expect("absolute path")
+        AbsolutePathBuf::from_absolute_path(absolute_path_string(path)).expect("absolute path")
     }
 
     #[test]
@@ -1419,6 +1456,27 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_fs_get_metadata() -> Result<()> {
+        let request = ClientRequest::FsGetMetadata {
+            request_id: RequestId::Integer(9),
+            params: v2::FsGetMetadataParams {
+                path: absolute_path("tmp/example"),
+            },
+        };
+        assert_eq!(
+            json!({
+                "method": "fs/getMetadata",
+                "id": 9,
+                "params": {
+                    "path": absolute_path_string("tmp/example")
+                }
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_list_experimental_features() -> Result<()> {
         let request = ClientRequest::ExperimentalFeatureList {