Skip missing paths in filesystem sandbox entries (#34598)

## Why

Default read-only protections for project metadata should apply when paths such
as `.git`, `.agents`, and `.codex` exist, without causing sandbox setup to
materialize missing paths as ACL targets.

## What changed

- Add an optional `missing_path_behavior` to filesystem sandbox entries and
  mark default project-metadata protections with `skip`.
- Preserve the behavior through permission transforms and exec/MCP protocol
  serialization while keeping existing path wire variants stable.
- Ignore skip-missing entries when projecting configuration or Windows sandbox
  overrides, while retaining explicit metadata carveouts.

## Testing

- Cover protocol round trips for path and special-path entries.
- Verify default metadata protections and Windows explicit carveout handling.

GitOrigin-RevId: 6df13dadacdd131c44aab9f15a967c81051355c1

codex-rs/tui/src/app_server_session.rs

@@ -2230,10 +2230,12 @@ mod tests {
                             value: FileSystemSpecialPath::Root,
                         },
                         access: FileSystemAccessMode::Read,
+                        missing_path_behavior: None,
                     },
                     FileSystemSandboxEntry {
                         path: FileSystemPath::Path { path: extra_root },
                         access: FileSystemAccessMode::Write,
+                        missing_path_behavior: None,
                     },
                 ],
                 glob_scan_max_depth: None,
@@ -2258,12 +2260,14 @@ mod tests {
                             value: FileSystemSpecialPath::Root,
                         },
                         access: FileSystemAccessMode::Read,
+                        missing_path_behavior: None,
                     },
                     FileSystemSandboxEntry {
                         path: FileSystemPath::Special {
                             value: FileSystemSpecialPath::ProjectRoots { subpath: None },
                         },
                         access: FileSystemAccessMode::Write,
+                        missing_path_behavior: None,
                     },
                 ],
                 glob_scan_max_depth: None,