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/core/src/config/mod.rs

@@ -2410,6 +2410,7 @@ fn apply_managed_filesystem_constraints(
                     pattern: deny_read.as_str().to_string(),
                 },
                 access: codex_protocol::permissions::FileSystemAccessMode::Deny,
+                missing_path_behavior: None,
             }
         } else {
             let Ok(path) = AbsolutePathBuf::try_from(deny_read.as_str()) else {
@@ -2418,6 +2419,7 @@ fn apply_managed_filesystem_constraints(
             codex_protocol::permissions::FileSystemSandboxEntry {
                 path: codex_protocol::permissions::FileSystemPath::Path { path },
                 access: codex_protocol::permissions::FileSystemAccessMode::Deny,
+                missing_path_behavior: None,
             }
         };
         if !file_system_sandbox_policy