fix(mcp): convert dttm cutoff to ISO string so filters_applied validates

The injected 7-day default filter used a datetime object as the value,
but ActionLogFilter.value only allows str|int|float|bool|list. Pydantic
rejects the datetime when building the filters_applied list in
ActionLogList, causing a ValidationError on every call that triggered
the default filter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>