fix(validator): refuse to overwrite corrupt PATS_FILE in save_pat / remove_pat

[ebios-star]

Summary

Closes #781.

`_read_file` in gittensor/validator/pat_storage.py was swallowing `(json.JSONDecodeError, OSError)` and returning `[]` on every failure, so a partial-write or on-disk corruption of `data/miner_pats.json` produced this sequence:

1. `_read_file()` silently returns `[]`.
2. `save_pat()` appends one fresh entry to the empty list and writes it.
3. The atomic `os.replace` overwrites the corrupt file with a single-entry list — every previously stored PAT is gone forever.

The read path keeps its graceful-degradation contract (`load_all_pats` and `get_pat_by_uid` still return empty/None on corrupt files so the validator scoring round can continue), but the write path now refuses to overwrite an unreadable file:

• `_read_file(*, raise_on_corrupt=False)` is the new signature.
• Read callers keep the default and additionally log a warning so the corruption is at least visible in operator logs.
• `save_pat` and `remove_pat` pass `raise_on_corrupt=True`; the `json.JSONDecodeError` / `OSError` now propagates up to the bittensor axon handler, which surfaces it to the broadcasting miner as a retriable error rather than acknowledging a silent data-loss write.

Related Issues

Closes #781.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• New `TestCorruptFileRefusesToOverwrite` covers four invariants:
  • `save_pat` raises `json.JSONDecodeError` when file is corrupt.
  • The on-disk corrupt bytes are preserved verbatim after a refused save.
  • `remove_pat` raises `json.JSONDecodeError` when file is corrupt.
  • `load_all_pats` still returns `[]` for a corrupt file (read-path graceful-degradation contract preserved).
• The pre-existing `test_load_handles_corrupt_file` is unchanged and still passes.
• `uv run --extra dev pytest tests/` — 447 tests pass (443 baseline + 4 new).

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[ebios-star]

@anderdc @LandynDev — friendly ping for review when you have a moment.

Closes #781. pat_storage._read_file was silently swallowing json.JSONDecodeError and returning []; the next save_pat then overwrote the corrupt file with a single fresh entry, permanently losing every previously stored PAT.

Read paths (load_all_pats, get_pat_by_uid) keep their graceful-degradation contract. Write paths (save_pat, remove_pat) now propagate the JSONDecodeError so the bittensor axon surfaces a retriable error to the broadcasting miner instead of acknowledging a silent data-loss write. The existing test_load_handles_corrupt_file is unchanged. 4 new regression tests including the on-disk-preservation invariant.

Happy to rebase if needed.