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/chatwidget/tests/permissions.rs

@@ -44,7 +44,7 @@ fn app_server_workspace_write_profile(extra_root: AbsolutePathBuf) -> Permission
                     missing_path_behavior: None,
                 },
                 FileSystemSandboxEntry {
-                    path: FileSystemPath::Path { path: extra_root },
+                    path: extra_root.into(),
                     access: FileSystemAccessMode::Write,
                     missing_path_behavior: None,
                 },
@@ -368,7 +368,7 @@ async fn preset_matching_does_not_treat_non_cwd_writable_profile_as_read_only()
                 },
                 FileSystemSandboxEntry {
                     path: FileSystemPath::Path {
-                        path: test_path_buf("/tmp/writable").abs(),
+                        path: test_path_buf("/tmp/writable").abs().into(),
                     },
                     access: FileSystemAccessMode::Write,
                     missing_path_behavior: None,