fix: load tiles during double-tap-and-drag (one finger) zoom gesture

[ProstoSanja]

fix: load tiles during double-tap-and-drag (one finger) zoom gesture

Problem

The double-tap-and-drag zoom gesture (double-tap, hold, then drag to zoom with
one finger — InteractiveFlag.doubleTapDragZoom) doesn't load new tiles while
the gesture is ongoing. The map zooms visually, but TileLayer keeps showing
the stale/upscaled/missing tiles and only loads the correct ones once the finger is
released. Pinch-to-zoom is unaffected by this behaviour.

Root cause

TileLayer only loads tiles in response to events on
MapController.mapEventStream. Every gesture-driven camera change flows through
MapControllerImpl.moveRaw, which builds its event via
MapEventWithMove.fromSource(...) (lib/src/gestures/map_events.dart):

switch (source) {
  ...
  MapEventSource.onDrag ||
  MapEventSource.onMultiFinger ||   // pinch zoom
  MapEventSource.mapController ||
  MapEventSource.keyboard ||
  MapEventSource.custom            => MapEventMove(...),
  _                                => null,   // ← doubleTapHold lands here
}

MapInteractiveViewer._handleDoubleTapHold drives the gesture with
moveRaw(..., source: MapEventSource.doubleTapHold), but doubleTapHold isn't
in the switch, so fromSource returns null and no MapEvent is emitted
for the whole gesture.

The existing //! PREVENTS TILE LOADING IF SOURCE NOT CONFIGURED IN CONSTRUCTOR
comment in moveRaw points at exactly this gap.

Testing

• Added test/map/map_events_test.dart, asserting fromSource(... source: doubleTapHold) returns a MapEventMove (with onMultiFinger as a baseline).

AI usage disclosure

• I have identified regressive behaviour myself in the application I am developing, but I used Claude Code to find the cause of the bug and analyse the code for potential fixes.
• Claude Code has been used to generate a test file
• I have manually verified the solution myself in the application I am developing.