feat: allow seeding the monitor resume position

[cconstab]

The monitor verb is monitor:<epochMillis> <regex>: the server replays notifications received after epochMillis. last_received_time was a class attribute with no way to set it at construction, so a caller that rebuilds its AtClient after a dropped monitor (a common resilience pattern) always sent monitor:0 — replaying the entire retained backlog or, depending on server retention, missing notifications that arrived while disconnected.

Changes

• last_received_time is now an AtMonitorConnection constructor parameter (default 0, behaviour unchanged) and is passed through AtClient.start_monitor(regex, last_received_time=...) — same construction-time-only semantics as the existing regex parameter.
• Monitor command construction extracted into _build_monitor_command() so the resume position is testable without a connection (same pattern as _is_shared_key_notification, fix: detect shared-key notifications (call to_string()) #524).
• The touched __init__ signature is wrapped and PEP8-spaced so the changed lines are flake8-clean.

Reference Dart SDK parity: Dart's monitor does exactly this — stayConnected() fetches getLastNotificationTime() on every reconnect and issues the monitor command via MonitorVerbBuilder..lastNotificationTime (packages/at_client/lib/src/manager/monitor.dart). Dart additionally persists the last received notification in the local keystore (local:lastreceivednotification..., gated by fetchOfflineNotifications); in at_python persistence is left to the caller, which this parameter enables. The internal heartbeat-driven restart already resumes correctly (it reuses the connection whose last_received_time is updated per notification in _run); this parameter covers the rebuild-the-client path.

Tests (test/monitor_resume_test.py, network-free): default emits monitor:0; a seeded position appears in the monitor command; AtClient.start_monitor passes the seed through to the connection.

Independent of #529 (no shared hunks; the test mocks start_heart_beat so it is thread-free either way).