fs: support pruning hidden directories during walks (#31570)

Why

Filtering hidden directories after a walk is too late: their descendants
consume traversal limits, and canonical directory deduplication can let
a hidden path claim a target before a visible symlink reaches it.

What this changes

- Add an optional pruneHiddenDirectories walk option. It defaults to
false and is omitted from the wire when disabled.
- Return hidden directory entries, but do not traverse them or add their
canonical identities to the visited set.
- Cover the visible-symlink-to-hidden-directory case through both local
and remote filesystem implementations.

This is the small filesystem prerequisite for #31566. Skill-specific
behavior remains in that PR.

codex-rs/exec-server/tests/file_system/shared.rs

@@ -402,6 +402,7 @@ async fn file_system_walk_returns_a_bounded_tree(
                 max_directories: 10,
                 max_entries: 10,
                 follow_directory_symlinks: false,
+                prune_hidden_directories: false,
             },
             /*sandbox*/ None,
         )
@@ -447,6 +448,7 @@ async fn file_system_walk_returns_a_bounded_tree(
                 max_directories: 10,
                 max_entries: 10,
                 follow_directory_symlinks: false,
+                prune_hidden_directories: false,
             },
             /*sandbox*/ None,
         )
@@ -469,6 +471,7 @@ async fn file_system_walk_returns_a_bounded_tree(
                 max_directories: 1,
                 max_entries: 10,
                 follow_directory_symlinks: false,
+                prune_hidden_directories: false,
             },
             /*sandbox*/ None,
         )
@@ -491,6 +494,7 @@ async fn file_system_walk_returns_a_bounded_tree(
                 max_directories: 10,
                 max_entries: 1,
                 follow_directory_symlinks: false,
+                prune_hidden_directories: false,
             },
             /*sandbox*/ None,
         )
@@ -535,6 +539,7 @@ async fn file_system_walk_honors_read_sandbox(
                 max_directories: 2,
                 max_entries: 2,
                 follow_directory_symlinks: false,
+                prune_hidden_directories: false,
             },
             Some(&sandbox),
         )