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/tui/src/app_server_session.rs

@@ -2583,7 +2583,7 @@ mod tests {
                         missing_path_behavior: None,
                     },
                     FileSystemSandboxEntry {
-                        path: FileSystemPath::Path { path: extra_root },
+                        path: extra_root.into(),
                         access: FileSystemAccessMode::Write,
                         missing_path_behavior: None,
                     },