[IMPROVEMENT] Malformed patterns field aborts guardrail rule loading instead of degrading like other fields

[Beloved1310]

Current state

Malformed field	Behaviour
Invalid regex in patterns	That pattern is skipped, rule still loads
Unrecognised action	That rule is skipped, file still loads
keywords: null	Normalised to an empty list, rule still loads
patterns: null	Uncaught TypeError aborts loading of the entire guardrails.yml

The failure originates at platform/guardrails/rules.py#L93,
where raw.get("patterns", []) returns None because the default applies only to an
absent key, not to a present key with an empty value.

The blast radius is wider than one file failing to parse. Nothing guards the call:
the only try/except in _parse_rule is the except re.error around re.compile,
which is never entered because the exception fires while evaluating the iterable, and
load_rules's own handler wraps yaml.safe_load, which has already completed by
then. All three callers invoke load_rules() bare. Because the exception propagates
before _engine is assigned, get_guardrail_engine() raises again on every
subsequent call rather than failing once.

Desired state

Malformed field	Behaviour
patterns: null	The rule is skipped and the reason is logged
patterns set to a non-list type	The rule is skipped and the reason is logged
Any other rule in the file	Unaffected, loading continues

The obvious fix is raw.get("patterns") or [], matching the keywords handling two
lines below.

[tmohanvamsi]

I’d like to take this. I’ll validate null/non-list patterns, skip only the malformed rule with a log message, and add regression tests confirming other rules still load. Is that scope correct?

[Beloved1310]

I am working on it. @tmohanvamsi