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/windows-sandbox-rs/src/deny_read_resolver.rs

@@ -226,7 +226,9 @@ mod tests {
     fn unreadable_path_entry(path: PathBuf) -> FileSystemSandboxEntry {
         FileSystemSandboxEntry {
             path: FileSystemPath::Path {
-                path: AbsolutePathBuf::from_absolute_path(path).expect("absolute path"),
+                path: AbsolutePathBuf::from_absolute_path(path)
+                    .expect("absolute path")
+                    .into(),
             },
             access: FileSystemAccessMode::Deny,
             missing_path_behavior: None,

codex-rs/windows-sandbox-rs/src/resolved_permissions.rs

@@ -333,28 +333,28 @@ mod tests {
             FileSystemSandboxPolicy::restricted(vec![
                 FileSystemSandboxEntry {
                     path: FileSystemPath::Path {
-                        path: first.clone(),
+                        path: first.clone().into(),
                     },
                     access: FileSystemAccessMode::Write,
                     missing_path_behavior: None,
                 },
                 FileSystemSandboxEntry {
                     path: FileSystemPath::Path {
-                        path: second.clone(),
+                        path: second.clone().into(),
                     },
                     access: FileSystemAccessMode::Write,
                     missing_path_behavior: None,
                 },
                 FileSystemSandboxEntry {
                     path: FileSystemPath::Path {
-                        path: first.join(".git"),
+                        path: first.join(".git").into(),
                     },
                     access: FileSystemAccessMode::Deny,
                     missing_path_behavior: None,
                 },
                 FileSystemSandboxEntry {
                     path: FileSystemPath::Path {
-                        path: second.join(".git"),
+                        path: second.join(".git").into(),
                     },
                     access: FileSystemAccessMode::Deny,
                     missing_path_behavior: None,