fix(quickadapter): catch UnicodeDecodeError in journal tail probe

fix(quickadapter): catch UnicodeDecodeError in journal tail probe

Codex inline review (P2) on PR #102 flagged that
`_optuna_journal_has_corrupt_tail` catches only
`json.JSONDecodeError` from `json.loads(last_line)`, but
`json.loads` raises `UnicodeDecodeError` (subclass of
`ValueError`, NOT of `JSONDecodeError`) when the trailing record
contains invalid UTF-8 bytes — a common crash pattern when
`fsync` is interrupted mid-multibyte. The exception escapes the
helper, propagates out of `optuna_create_storage` (the helper
runs BEFORE the recoverable try/except), reaches
`optuna_create_study`'s broad outer handler, and reproduces the
silent-HPO-skip symptom under a different corruption class.

Empirically reproduced:

  >>> json.loads(b'\\xc3\\x28')
  UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc3 in
  position 0: invalid continuation byte

Broaden the except clause from `json.JSONDecodeError` to
`ValueError` — the common parent of both `JSONDecodeError` and
`UnicodeDecodeError` — so the helper treats any
`json.loads`-unparseable trailing record as corruption and routes
it through the same quarantine path. Other `ValueError` subclasses
are not plausibly raised by `json.loads` on bytes input.

Reproducer at `/tmp/quickadapter-tests/test_optuna_journal_quarantine.py`
extended from 17 to 19 scenarios (Class C4 detection + end-to-end
quarantine on invalid UTF-8). All pass.