fix: validate complete-day monitoring coverage

src/fenris/monitoring_periods.py

@@ -9,7 +9,7 @@ Key contracts:
 - End causes: user_disabled, migrated, unknown_gap
 """
 import sqlite3
-from datetime import datetime
+from datetime import datetime, timezone
 
 
 def ensure_period_open(conn: sqlite3.Connection, run_time: datetime) -> None:
@@ -82,6 +82,46 @@ def is_inside_period(conn: sqlite3.Connection, ts: datetime) -> bool:
     return cursor.fetchone() is not None
 
 
+def interval_within_one_monitoring_period(
+    conn: sqlite3.Connection,
+    start: str | datetime,
+    end: str | datetime,
+) -> bool:
+    """Return whether one monitoring period contains the full interval.
+
+    Compare timestamps as UTC instants. Malformed timestamps fail closed.
+    """
+    try:
+        interval_start = as_utc_datetime(start)
+        interval_end = as_utc_datetime(end)
+    except (TypeError, ValueError, OverflowError):
+        return False
+    if interval_end <= interval_start:
+        return False
+
+    for started_at, ended_at in conn.execute(
+        "SELECT started_at, ended_at FROM monitoring_periods"
+    ):
+        try:
+            period_start = as_utc_datetime(started_at)
+            period_end = as_utc_datetime(ended_at) if ended_at is not None else None
+        except (TypeError, ValueError, OverflowError):
+            continue
+        if period_start <= interval_start and (
+            period_end is None or interval_end <= period_end
+        ):
+            return True
+    return False
+
+
+def as_utc_datetime(value: str | datetime) -> datetime:
+    """Parse a timestamp and normalize it to an aware UTC datetime."""
+    parsed = value if isinstance(value, datetime) else datetime.fromisoformat(value)
+    if parsed.tzinfo is None:
+        parsed = parsed.replace(tzinfo=timezone.utc)
+    return parsed.astimezone(timezone.utc)
+
+
 def wall_clock_in_periods(
     conn: sqlite3.Connection,
     start: datetime,