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/exec/src/event_processor_with_human_output_tests.rs

@@ -143,12 +143,14 @@ fn summarizes_managed_workspace_write_permission_profile() {
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path { path: cwd.clone() },
                 access: FileSystemAccessMode::Write,
+                missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path {
                     path: cache_root.clone(),
                 },
                 access: FileSystemAccessMode::Write,
+                missing_path_behavior: None,
             },
         ]),
         NetworkSandboxPolicy::Restricted,