Preserve filesystem permission path conventions (#39084)

## Why

Filesystem permission paths can use a convention that differs from the host
running Codex. Converting them immediately to native absolute paths can change
the meaning of ambiguous paths such as `/C:/secret` or Windows UNC paths.

## What changed

- Store literal filesystem permission paths as `PathUri` values through the
  runtime policy and execution protocol.
- Keep legacy string-based serialization at explicit protocol boundaries and
  reject conversions that cannot be represented losslessly.
- Encode native paths as opaque URIs when a normal file URI would imply the
  wrong path convention.

## Testing

Added coverage for cross-platform and ambiguous path round trips, UNC path
variants, permission-profile serialization, and deny-policy enforcement.

GitOrigin-RevId: 5247713796d1f2bb4e02f94eb9fc82d4698060f0

codex-rs/exec/src/event_processor_with_human_output_tests.rs

@@ -141,13 +141,13 @@ fn summarizes_managed_workspace_write_permission_profile() {
     let profile = PermissionProfile::from_runtime_permissions(
         &FileSystemSandboxPolicy::restricted(vec![
             FileSystemSandboxEntry {
-                path: FileSystemPath::Path { path: cwd.clone() },
+                path: cwd.clone().into(),
                 access: FileSystemAccessMode::Write,
                 missing_path_behavior: None,
             },
             FileSystemSandboxEntry {
                 path: FileSystemPath::Path {
-                    path: cache_root.clone(),
+                    path: cache_root.clone().into(),
                 },
                 access: FileSystemAccessMode::Write,
                 missing_path_behavior: None,