Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@playbooks/robusta_playbooks/common_actions.py`:
- Around line 110-111: The current logging in customise_finding logs full
labels_to_inject which can leak sensitive metadata; change the call in the
customise_finding block so you do not log full label values—either downgrade to
logging.debug or log only the label keys (e.g., list(labels_to_inject.keys()))
before calling event.inject_finding_labels(labels_to_inject); keep the injection
via event.inject_finding_labels unchanged but ensure only non-sensitive key
names or a debug-level message is emitted.
- Around line 102-103: customise_finding currently calls
re.fullmatch(rule.regex, source_value) which will raise re.error for malformed
user regexes; wrap the re.fullmatch call in a try/except catching re.error, log
the invalid pattern and exception (including rule.regex and any identifying rule
id) using the existing logger, then continue to the next rule so the bad pattern
is skipped instead of crashing customise_finding.

In `@src/robusta/core/model/events.py`:
- Around line 158-161: The issue is that Finding uses a shared mutable default
FindingSubject which causes inject_finding_labels to mutate a shared labels
dict; fix by changing the Finding constructor signature (class Finding) to use
subject: Optional[FindingSubject] = None instead of a default instance, and
inside the constructor create a fresh FindingSubject() when subject is None
(assign self.subject = subject or FindingSubject()); keep inject_finding_labels
as-is but this ensures each Finding has its own subject.labels so .update()
won't bleed labels across Findings.

---

Nitpick comments:
In `@tests/test_customise_finding_relabel.py`:
- Around line 179-208: Add a test that verifies
customise_finding/FindingOverrides tolerate an invalid regex by creating a
FindingSubject (use _run or _make_event) and a rule where "regex" is malformed
(e.g. unmatched bracket), then call customise_finding or _run with that rule and
assert the function does not raise and the finding's labels remain unchanged
(e.g. existing "team" label stays the same or target_label is not added);
reference customise_finding, FindingOverrides, _run, _make_event and
FindingSubject to locate where to add the test and ensure the bad rule is
skipped rather than crashing.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/robusta/core/reporting/base.py`:
- Around line 261-272: Update the subject parameter to accept None by changing
its type to Optional[FindingSubject] and make its default value None (i.e.,
subject: Optional[FindingSubject] = None) so the existing guard "if subject is
None: subject = FindingSubject()" is authoritative; this also removes the
mutable-default issue—adjust any import/type hints as needed to reference
Optional and keep the runtime logic in the constructor (the parameter and the
guard in the function/method that declares subject and uses FindingSubject).