fix Issue #2418: parse_rfc3339 calling .groups() on None

[omavashia2005]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixes a runtime bug in parse_rfc3339() where the function would call .groups() on a None result if the regex did not match. Previously caused AttributeError: 'NoneType' object has no attribute 'groups'

Which issue(s) this PR fixes:

Fixes #2418

Special notes for your reviewer:

Includes unit test test_2_parse_rfc3339 with malformed datetime strings to verify fix and prevent regressions.

Does this PR introduce a user-facing change?

Improved RFC3339 datetime parsing logic in kube config loading. Invalid strings now raise a descriptive 
ValueError instead of a raw AttributeError.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

NA

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[roycaihw]

/assign

[afshin-deriv]

I think in the current implementation of parse_rfc3339, the _re_rfc3339 regex accept several things that RFC 3339 forbids, also make some required pieces optional - as a result, in some cases datetime.datetime will crash with a meaningful messages:

for example, _re_rfc3339 accept 25 as hour although RFC 3339 ABNF says:
time-hour = 2DIGIT ; 00-23:

>>> from kubernetes.base.config import dateutil

>>> s = "2025-10-04t25:00:00"
>>> dateutil.parse_rfc3339(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/afshinpaydar/go/src/k8s.io/python/kubernetes/base/config/dateutil.py", line 74, in parse_rfc3339
    return datetime.datetime(
           ^^^^^^^^^^^^^^^^^^
ValueError: hour must be in 0..23
>>>

but in this changes, the catch all section only shows Error in RFC339 Date Formatting .. message, it seems misleading to me as the regex is not quite strict/correct:

>>> from kubernetes.base.config import dateutil
>>> s = "2025-10-04Z20:00:00"
>>> dateutil.parse_rfc3339(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/afshinpaydar/go/src/k8s.io/python/kubernetes/base/config/dateutil.py", line 60, in parse_rfc3339
    raise ValueError(f"Error in RFC339 Date Formatting {s}")
ValueError: Error in RFC339 Date Formatting 2025-10-04Z20:00:00

maybe something like below output would be more helpful for users:

ValueError: Invalid RFC 3339 ... , Expected ...

IMHO, I really like the idea of doing less and enable more:

-    groups = _re_rfc3339.search(s).groups()
+    m = _re_rfc3339.fullmatch(s.strip())
+    if m is None:
+        raise ValueError(
+            f"Invalid RFC3339 datetime: {s!r} "
+            "(expected YYYY-MM-DDTHH:MM:SS[.frac][Z|±HH:MM])"
+        )
+    groups = m.groups()