[mojo] Expose --check mode on mojo format - #6888

What this PR does

This PR delivers the --check mode feature in two layers:

Layer 1 — Working TODAY: utils/mojo-format-check.py

A new script that implements the full requested behavior right now, without
requiring any changes to the closed-source mojo binary.

It works by calling mblack directly — the same underlying formatter that
mojo format uses internally — but in check mode:

# Check formatting without modifying any files
python3 utils/mojo-format-check.py src/ tests/ examples/

# Exit codes (match black/mblack spec):
#   0   → all files already formatted
#   1   → one or more files would change (files NOT modified)
#   123 → internal formatter error

This is already wired up to:

• pixi run format-check (via mojo/pixi.toml)
• A pre-push pre-commit hook (via .pre-commit-config.yaml)

Layer 2 — Needs Modular internal work: mojo format --check

The real fix the issue requests — mojo format --check working as a native
CLI flag — requires modifying the mojo driver binary (C++ source in the
private KGEN repo). This PR cannot do that, but the implementation is
straightforward:

cl::opt<bool> FormatCheck(
    "check",
    cl::desc("Don't write files; exit 0 if formatted, 1 if any would change."),
    cl::sub(FormatSubCmd));

cl::opt<bool> FormatDiff(
    "diff",
    cl::desc("Don't write files; show a unified diff instead."),
    cl::sub(FormatSubCmd));

if (FormatCheck) mblack_args.push_back("--check");
if (FormatDiff)  mblack_args.push_back("--diff");
// ... existing --quiet, --line-length ...
int rc = RunSubprocess(MBlackPath, mblack_args);
exit(rc);  // CRITICAL: pass exit code through unchanged — do NOT remap

Files changed