Add a bounded filesystem walk RPC (#29841)

Stack 1 of 3. Follow-ups: #29842 and #29844.

## What changes

Adds a general bounded `fs/walk` operation to the exec server.

The operation returns file and directory entries plus recoverable
per-path errors. It skips symlinks, preserves the existing filesystem
sandbox routing, and enforces depth, directory, entry, and response-size
limits.

This PR only defines and wires the filesystem operation. It does not
change any callers yet.

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

@@ -2,6 +2,8 @@ use std::collections::HashMap;
 
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
 use codex_file_system::FileSystemSandboxContext;
+pub use codex_file_system::WalkOptions;
+pub use codex_file_system::WalkOutcome;
 use codex_network_proxy::ManagedNetworkSandboxContext;
 use codex_protocol::config_types::ShellEnvironmentPolicyInherit;
 use codex_shell_command::shell_detect::DetectedShell;
@@ -31,6 +33,7 @@ pub const FS_CREATE_DIRECTORY_METHOD: &str = "fs/createDirectory";
 pub const FS_GET_METADATA_METHOD: &str = "fs/getMetadata";
 pub const FS_CANONICALIZE_METHOD: &str = "fs/canonicalize";
 pub const FS_READ_DIRECTORY_METHOD: &str = "fs/readDirectory";
+pub const FS_WALK_METHOD: &str = "fs/walk";
 pub const FS_REMOVE_METHOD: &str = "fs/remove";
 pub const FS_COPY_METHOD: &str = "fs/copy";
 /// JSON-RPC request method for executor-side HTTP requests.
@@ -370,6 +373,16 @@ pub struct FsReadDirectoryResponse {
     pub entries: Vec<FsReadDirectoryEntry>,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct FsWalkParams {
+    pub path: PathUri,
+    pub options: WalkOptions,
+    pub sandbox: Option<FileSystemSandboxContext>,
+}
+
+pub type FsWalkResponse = WalkOutcome;
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct FsRemoveParams {