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-server/tests/exec_process.rs

@@ -205,12 +205,14 @@ async fn remote_process_keeps_sandbox_helper_visible_with_restricted_reads() ->
                 value: FileSystemSpecialPath::Minimal,
             },
             access: FileSystemAccessMode::Read,
+            missing_path_behavior: None,
         },
         FileSystemSandboxEntry {
             path: FileSystemPath::Special {
                 value: FileSystemSpecialPath::project_roots(/*subpath*/ None),
             },
             access: FileSystemAccessMode::Read,
+            missing_path_behavior: None,
         },
     ]);
     let sandbox = FileSystemSandboxContext::from_permission_profile_with_cwd(
@@ -272,12 +274,14 @@ async fn remote_tty_process_uses_configured_sandbox_helper_with_hostile_path() -
                 value: FileSystemSpecialPath::Minimal,
             },
             access: FileSystemAccessMode::Read,
+            missing_path_behavior: None,
         },
         FileSystemSandboxEntry {
             path: FileSystemPath::Special {
                 value: FileSystemSpecialPath::project_roots(/*subpath*/ None),
             },
             access: FileSystemAccessMode::Read,
+            missing_path_behavior: None,
         },
     ]);
     let sandbox = FileSystemSandboxContext::from_permission_profile_with_cwd(
@@ -332,6 +336,7 @@ async fn remote_process_preserves_empty_workspace_roots() -> Result<()> {
             value: FileSystemSpecialPath::project_roots(/*subpath*/ None),
         },
         access: FileSystemAccessMode::Read,
+        missing_path_behavior: None,
     }]);
     let mut sandbox = FileSystemSandboxContext::from_permission_profile_with_cwd(
         PermissionProfile::from_runtime_permissions(&policy, NetworkSandboxPolicy::Restricted),