fix(hermes_time): implement reset_cache() referenced in docstrings (#41728)

The module docstring and get_timezone()/cache comments documented a
reset_cache() helper for forcing tz re-resolution after config changes,
but the function was never defined — doc-followers calling it hit
AttributeError. Adds the helper to clear the cached tz state.

Surfaced in #32043.

hermes_time.py

@@ -88,6 +88,19 @@ def get_timezone() -> Optional[ZoneInfo]:
     return _cached_tz
 
 
+def reset_cache() -> None:
+    """Clear the cached timezone so the next call re-resolves it.
+
+    Call this after the configured timezone may have changed (e.g. after a
+    config edit or ``HERMES_TIMEZONE`` update) to force ``get_timezone()`` /
+    ``now()`` to read the new value instead of the value cached at first use.
+    """
+    global _cached_tz, _cached_tz_name, _cache_resolved
+    _cached_tz = None
+    _cached_tz_name = None
+    _cache_resolved = False
+
+
 def now() -> datetime:
     """
     Return the current time as a timezone-aware datetime.