Remove fs/join and fs/parent from exec-server protocol (#27700)

## Summary

Path composition is already handled by `PathUri`, leaving `fs/join` and
`fs/parent` as redundant exec-server protocol surface. Because
app-server and exec-server are deployed atomically, these obsolete
methods can be removed without a compatibility shim.

This removes the protocol constants and payloads, public client APIs,
server registrations and handlers, and endpoint-only tests. Existing
in-process `PathUri` join/parent coverage remains.

## Validation

- `just test -p codex-exec-server` (215 passed, 2 skipped)

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

@@ -25,10 +25,6 @@ use crate::protocol::FsCreateDirectoryParams;
 use crate::protocol::FsCreateDirectoryResponse;
 use crate::protocol::FsGetMetadataParams;
 use crate::protocol::FsGetMetadataResponse;
-use crate::protocol::FsJoinParams;
-use crate::protocol::FsJoinResponse;
-use crate::protocol::FsParentParams;
-use crate::protocol::FsParentResponse;
 use crate::protocol::FsReadDirectoryParams;
 use crate::protocol::FsReadDirectoryResponse;
 use crate::protocol::FsReadFileParams;
@@ -270,22 +266,6 @@ impl ExecServerHandler {
         self.file_system.canonicalize(params).await
     }
 
-    pub(crate) async fn fs_join(
-        &self,
-        params: FsJoinParams,
-    ) -> Result<FsJoinResponse, JSONRPCErrorError> {
-        self.require_initialized_for("filesystem")?;
-        self.file_system.join(params).await
-    }
-
-    pub(crate) async fn fs_parent(
-        &self,
-        params: FsParentParams,
-    ) -> Result<FsParentResponse, JSONRPCErrorError> {
-        self.require_initialized_for("filesystem")?;
-        self.file_system.parent(params).await
-    }
-
     pub(crate) async fn fs_read_directory(
         &self,
         params: FsReadDirectoryParams,