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/models.rs

@@ -89,15 +89,19 @@ impl FileSystemPermissions {
     ) -> Self {
         let mut entries = Vec::new();
         if let Some(read) = read {
-            entries.extend(read.into_iter().map(|path| FileSystemSandboxEntry {
-                path: FileSystemPath::Path { path },
-                access: FileSystemAccessMode::Read,
+            entries.extend(read.into_iter().map(|path| {
+                FileSystemSandboxEntry::new(
+                    FileSystemPath::Path { path },
+                    FileSystemAccessMode::Read,
+                )
             }));
         }
         if let Some(write) = write {
-            entries.extend(write.into_iter().map(|path| FileSystemSandboxEntry {
-                path: FileSystemPath::Path { path },
-                access: FileSystemAccessMode::Write,
+            entries.extend(write.into_iter().map(|path| {
+                FileSystemSandboxEntry::new(
+                    FileSystemPath::Path { path },
+                    FileSystemAccessMode::Write,
+                )
             }));
         }
         Self {
@@ -636,12 +640,12 @@ impl From<&FileSystemSandboxPolicy> for FileSystemPermissions {
         let entries = match value.kind {
             FileSystemSandboxKind::Restricted => value.entries.clone(),
             FileSystemSandboxKind::Unrestricted | FileSystemSandboxKind::ExternalSandbox => {
-                vec![FileSystemSandboxEntry {
-                    path: FileSystemPath::Special {
+                vec![FileSystemSandboxEntry::new(
+                    FileSystemPath::Special {
                         value: FileSystemSpecialPath::Root,
                     },
-                    access: FileSystemAccessMode::Write,
-                }]
+                    FileSystemAccessMode::Write,
+                )]
             }
         };
         Self {
@@ -2497,6 +2501,7 @@ mod tests {
                     pattern: "**/*.env".to_string(),
                 },
                 access: FileSystemAccessMode::Deny,
+                missing_path_behavior: None,
             }]);
         file_system_sandbox_policy.glob_scan_max_depth = Some(2);
 
@@ -2542,6 +2547,7 @@ mod tests {
                             value: FileSystemSpecialPath::Root,
                         },
                         access: FileSystemAccessMode::Write,
+                        missing_path_behavior: None,
                     }],
                     glob_scan_max_depth: NonZeroUsize::new(2),
                 },
@@ -2685,6 +2691,7 @@ mod tests {
             entries: vec![FileSystemSandboxEntry {
                 path: FileSystemPath::Path { path },
                 access: FileSystemAccessMode::Read,
+                missing_path_behavior: None,
             }],
             glob_scan_max_depth: NonZeroUsize::new(2),
         };