[codex] Migrate apply_patch to executor filesystem (#17027)

- Migrate apply-patch verification and application internals to use the
async `ExecutorFileSystem` abstraction from `exec-server`.
- Convert apply-patch `cwd` handling to `AbsolutePathBuf` through the
verifier/parser/handler boundary.

Doesn't change how the tool itself works.

codex-rs/exec-server/src/file_system.rs

@@ -39,6 +39,12 @@ pub type FileSystemResult<T> = io::Result<T>;
 pub trait ExecutorFileSystem: Send + Sync {
     async fn read_file(&self, path: &AbsolutePathBuf) -> FileSystemResult<Vec<u8>>;
 
+    /// Reads a file and decodes it as UTF-8 text.
+    async fn read_file_text(&self, path: &AbsolutePathBuf) -> FileSystemResult<String> {
+        let bytes = self.read_file(path).await?;
+        String::from_utf8(bytes).map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
+    }
+
     async fn write_file(&self, path: &AbsolutePathBuf, contents: Vec<u8>) -> FileSystemResult<()>;
 
     async fn create_directory(