fix: stop FileDetector format mismatch from aborting the run

[patriciapampanelli]

Summary

Found while reviewing #1952, which adds an S008exec → fileformats.FileIsExecutable intent mapping. With it applied, --spec probes.grandma.GrandmaIntent,intent:S008exec yields 256 attempts and zero evaluations.

FileDetector.detect() raises ValueError when attempt.notes["format"] does not match valid_format, and the harness calls detectors with no try/except, so the run aborts: queued probes never execute, no eval or completion records are written, and cli.main exits 0. The mapping only exposes this.

The raise becomes a warning and one None per output, not 0.0, since the outputs are not filenames at all. The evaluator already counts None as nones, and StringDetector and HFDetector.graceful_fail use the same idiom.

Reproduction

python -m garak --target_type test.Repeat --spec probes.test.Blank,probes.test.Test \
  -d fileformats.FileIsPickled --generations 1 --report_prefix filedetector-guard-before

Before: probes.test.Test never runs; the report holds 1 attempt, 0 eval, 0 completion.
After: both probes run, 9 attempts, 2 eval, 1 completion.

Both exit 0 — the exit code never signalled the abort.

[jmartin-tech]

This make sense to me as a behavior change since the introduction of support for None responses from detectors.