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/protocol/src/protocol.rs

@@ -4930,6 +4930,7 @@ mod tests {
                 value: FileSystemSpecialPath::Root,
             },
             access: FileSystemAccessMode::Read,
+            missing_path_behavior: None,
         }]);
         assert!(read_only.has_full_disk_read_access());
         assert!(!read_only.has_full_disk_write_access());
@@ -4940,6 +4941,7 @@ mod tests {
                 value: FileSystemSpecialPath::Root,
             },
             access: FileSystemAccessMode::Write,
+            missing_path_behavior: None,
         }]);
         assert!(writable.has_full_disk_read_access());
         assert!(writable.has_full_disk_write_access());
@@ -4970,10 +4972,12 @@ mod tests {
                     value: FileSystemSpecialPath::Root,
                 },
                 access: FileSystemAccessMode::Write,
+                missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path { path: blocked },
                 access: FileSystemAccessMode::Deny,
+                missing_path_behavior: None,
             },
         ]);
 
@@ -5021,16 +5025,19 @@ mod tests {
                     value: FileSystemSpecialPath::Minimal,
                 },
                 access: FileSystemAccessMode::Read,
+                missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Special {
                     value: FileSystemSpecialPath::project_roots(/*subpath*/ None),
                 },
                 access: FileSystemAccessMode::Write,
+                missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path { path: secret },
                 access: FileSystemAccessMode::Deny,
+                missing_path_behavior: None,
             },
         ]);
 
@@ -5089,14 +5096,17 @@ mod tests {
                     value: FileSystemSpecialPath::project_roots(/*subpath*/ None),
                 },
                 access: FileSystemAccessMode::Write,
+                missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path { path: docs },
                 access: FileSystemAccessMode::Read,
+                missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path { path: docs_public },
                 access: FileSystemAccessMode::Write,
+                missing_path_behavior: None,
             },
         ]);
 
@@ -5133,6 +5143,7 @@ mod tests {
                 path: external_write_path,
             },
             access: FileSystemAccessMode::Write,
+            missing_path_behavior: None,
         }]);
 
         let err = policy
@@ -6115,6 +6126,7 @@ mod tests {
                         pattern: "/tmp/private/**/*.txt".to_string(),
                     },
                     access: FileSystemAccessMode::Deny,
+                    missing_path_behavior: None,
                 },
             ])),
             model: "gpt-5".to_string(),