tests: cover plugin.py ZoneOn duration math via an extracted helper

[simons-plugins]

Context

PR #50 rewrote the standard Indigo ZoneOn sprinkler action handler. The new branch contains real logic (seconds → minutes conversion, clamp to maxZoneRunTime, max(1, round(...)) floor) that has zero direct test coverage because plugin.py historically can't be unit-tested — it needs the indigo runtime.

The conversion math itself is pure and doesn't need Indigo. PR review pointed out that extracting it into a module-level helper would close the only meaningful logic gap introduced by #50.

Proposed change

Pull the duration conversion out into a helper that can live in utils.py or device_handlers.py:

def compute_zone_duration_minutes(zone_max_seconds: int, cap_seconds: int) -> int:
    \"\"\"Compute the Netro API duration for a zone.

    Netro takes minutes on the wire; Indigo's zoneMaxDurations is in seconds.
    Returns at least 1 (Netro rejects 0) and never exceeds `cap_seconds`
    converted to minutes.
    \"\"\"
    seconds = min(zone_max_seconds, cap_seconds)
    return max(1, int(round(seconds / 60)))

Then plugin.py ZoneOn becomes:

duration_minutes = compute_zone_duration_minutes(
    zone_max_seconds, self.maxZoneRunTime
)

Test cases to add

• Basic conversion (3600s cap, 1800s zone → 30min)
• Cap lower than zone max (1800s cap, 3600s zone → 30min)
• Fractional minutes round correctly (90s → 2min via round)
• 30 seconds floors to 1 minute, not 0
• Zero zone max still returns 1 (even though ZoneOn refuses <= 0 upstream, this keeps the helper safe)

Scope

Small. Pure refactor + ~5 unit tests. Follow-up from #50 review, non-blocking.