fix: handle non-dict parsed JSON in firmware version manifest parser

[dholt]

Summary

• roles/nvidia-dgx-firmware/files/parse_manifest.py parse_versioning keeps the last json.loads-parseable line from the firmware container output, then indexes it with a string key (manifest_json['ErrorWritingVersioning']).
• If that last parseable line decodes to a non-dict (e.g. a JSON array), the string-key index raises TypeError. Only KeyError was caught, so the exception escaped and aborted the task.
• Guard that the parsed value is a dict before indexing; a non-dict now falls through to the existing "No JSON could be loaded" path and exits 1 cleanly instead of crashing.

Validation

• git diff --check origin/master...HEAD
• py_compile of the changed script
• Reproduced the crash before the fix (non-dict last line → uncaught TypeError) and confirmed the clean exit 1 after the fix.
• Added a self-contained regression test and ran it:
  python -m unittest discover -s roles/nvidia-dgx-firmware/tests -v → Ran 1 test ... OK
• Public sanitizer pass on the PR body and added diff lines.

Notes

• Behavior is unchanged for well-formed manifests; this only hardens the parser against malformed/non-dict input.
• No pre-existing tests covered this script/role; the new test uses only the standard library and a tempfile, no external fixtures.

[dholt]

@michael-balint Gentle ping on this one when you have a moment — it's a small, self-contained robustness fix for parse_manifest.py with a stdlib-only regression test. All 31 checks are green and it's mergeable. Thanks!