Switch stable tests to NixOS 25.11

[josh]

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[Copilot]

Pull request overview

This PR updates the stable NixOS version used for testing from 25.05 to 25.11. The repository uses internal/flake.lock with builtins.getFlake to pin dependencies for running test checks against both stable and unstable NixOS versions, as shown in the main flake.nix where mkChecks is called for both "stable" and "unstable" variants.

• Updated nixpkgs-stable reference from nixos-25.05 to nixos-25.11
• Regenerated flake.lock with new commit hash and metadata

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/flake.nix	Updates the nixpkgs-stable input URL from nixos-25.05 to nixos-25.11
internal/flake.lock	Updates the locked nixpkgs-stable metadata including commit hash, narHash, and lastModified timestamp

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