Fix unable to complete draw area due to wrong initial vertices

[andreia-ferreira]

Related to #3679

Some users in a Ground training in Lao PDR reported that they were not able to complete polygon drawing because the 'Complete' button was disabled and no error message was displayed. This was caused by an issue where a duplicated first vertex would be created by tapping 'Add point' twice at the start of drawing.
commitTentativeVertex guarded against re-committing the same spot with _isTooClose = _vertices.size > 1. But this caused the 'Add point' button to remain enabled after adding 1 vertex, which allowed for the duplication and caused an invalid final geometry which made it impossible to close the polygon.

This fix involves setting isTooClose to true after every committed vertex, which sets the 'Add Pointas disabled until the user moves the map again. There is also a small refactor inPolygonDrawingSessionImpl` to make sure the first vertex does stay in the correct place:

• added a separate class for committed vertices (committedVertices) and other for just the tentative vertex following the map center (cursor)
• setGeometry, removeLastVertex, redoLastVertex, and checkVertexIntersection were simplified to operate through the derived vertices.

Before

Screen_recording_20260713_120515.webm

After

Screen_recording_20260713_120709.webm

@shobhitagarwal1612 PTAL?

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.08%. Comparing base (8dbca61) to head (4dabb78).

Files with missing lines	Patch %	Lines
...lection/tasks/polygon/PolygonDrawingSessionImpl.kt	93.10%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3835      +/-   ##
============================================
- Coverage     67.08%   67.08%   -0.01%     
+ Complexity     1799     1796       -3     
============================================
  Files           405      405              
  Lines         10633    10629       -4     
  Branches       1396     1395       -1     
============================================
- Hits           7133     7130       -3     
+ Misses         2758     2757       -1     
  Partials        742      742              

Files with missing lines	Coverage Δ
...lection/tasks/polygon/PolygonDrawingSessionImpl.kt	93.75% <93.10%> (-0.49%)	⬇️

... and 1 file with indirect coverage changes

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