fix(map-drawing-terradraw): initialize drawing session only once on projection_changed

[zigzagdev]

Summary

Motivation

samples/map-drawing-terradraw/index.ts set up the entire drawing session (the TerraDraw instance, its select/deselect/change handlers, the export/upload/resize/delete/undo/redo button handlers, and a document.addEventListener('keydown', ...) rotate-on-r handler) inside a map.addListener('projection_changed', ...) callback.
projection_changed is not a fires-once-at-startup event — it can fire again later, e.g. when the map's rendering/map type changes.
Therefore this sample enables mapTypeControl: true, the built-in map type control is clickable at any time, including mid-drawing.

If projection_changed fired a second time:

• The live TerraDraw instance was silently replaced with a new one (without calling .stop() on the old one), wiping the user's in-progress drawing from the map without warning.
• A new document.addEventListener('keydown', ...) handler was added on top of the existing one, since addEventListener (unlike the .onclick = button handlers) doesn't overwrite previous listeners. Each additional firing stacked one more global keydown listener, so pressing r with a feature selected would duplicate the rotated feature once per prior map-type switch.

What I have done

Changed map.addListener('projection_changed', ...) to google.maps.event.addListenerOnce(map, 'projection_changed', ...) so the one-time setup logic can only ever run once, regardless of how many times projection_changed fires afterward.

Test Plans

• Manually verified the logic: with addListenerOnce, the setup block (TerraDraw instance creation, button handlers, and the keydown rotate listener) is guaranteed to execute exactly once per page load, so a subsequent projection_changed firing (e.g. from toggling the map type control) no longer replaces the active drawing session or registers an additional keydown listener.

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