Fix SonarCloud issues: simplify complex methods and clean up

[mfl28]

Fixes about 40 of the 61 open SonarCloud issues on master. No behavior changes.

Critical: all 7 methods that were too complex

• PNGMaskLoadStrategy: reading a single mask file moved into a per-import LoadContext (strategies keep per-call state in a context object). A small ObjectRegion class collects each object's bounds and category counts.
• PNGMaskSaveStrategy: converting a shape to a mask (toMask) and painting a mask (paint) are separate methods, which also removes the continues.
• COCOLoadStrategy: readRoot checks the file and its arrays, readAnnotation reads one annotation, and the results moved into the existing LoadContext record.
• COCORunLengthEncoding.decode: reading a single count moved into a CountReader.
• BoundingMaskView and EditorImagePaneView: the event-handler bodies became named methods.

Major

• MutableMask: fillCircle, fillPolygon and strokeLine return Optional<PixelRegion> (a new record) instead of an int[] or null. Two loops that changed their counter in the body are now while loops.
• CSVLoadStrategy: the nested try blocks became readRow and importRow.
• The three load exceptions are no longer nested in ImageAnnotationLoadStrategy; each has its own file.
• The number input's regex uses possessive quantifiers. It accepts the same input but can't backtrack.
• Fields of inner classes no longer hide outer ones (ResizeHandle.handleDragAnchor, VertexHandle.vertexEditing).
• main passes its arguments to launch.
• PVOC flag tags (truncated/occluded/difficult) are handled in their own method.

Minor: unnamed catch variables (11), a loop without continue in ImageAnnotationSaver, assertThrows instead of try/fail (4 tests), method references, an unused import, an unnecessary throws, and != instead of !(==).

Left out on purpose

• Assertion counts in long UI tests (S5961): they belong to a separate test-splitting change.
• The 5 issues in Controller: Move the autosave logic from Controller into AutosaveController #228 changes it heavily, so they follow after it's merged.
• MainView "monster class": too big a change for a cleanup.
• Hard-coded URIs (S1075): these are bundled resource paths.
• transient in BoundingMaskData: it deliberately keeps Gson from serializing masks by reflection.

The full suite (291 tests) passes locally.

[codecov]

Codecov Report

❌ Patch coverage is 86.31579% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.25%. Comparing base (509e580) to head (f1d0bac).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...fl28/boundingboxeditor/model/data/MutableMask.java	61.11%	4 Missing and 3 partials ⚠️
...b/mfl28/boundingboxeditor/ui/BoundingMaskView.java	76.66%	2 Missing and 5 partials ⚠️
...oundingboxeditor/model/io/PNGMaskLoadStrategy.java	93.18%	1 Missing and 5 partials ⚠️
...fl28/boundingboxeditor/ui/EditorImagePaneView.java	62.50%	4 Missing and 2 partials ⚠️
...undingboxeditor/model/io/ImageAnnotationSaver.java	63.63%	3 Missing and 1 partial ⚠️
...28/boundingboxeditor/model/io/CSVLoadStrategy.java	85.71%	0 Missing and 2 partials ⚠️
.../mfl28/boundingboxeditor/BoundingBoxEditorApp.java	0.00%	1 Missing ⚠️
...ingboxeditor/controller/ImageFolderController.java	0.00%	1 Missing ⚠️
...28/boundingboxeditor/model/data/ImageMetaData.java	0.00%	1 Missing ⚠️
...undingboxeditor/model/io/BoundingBoxPredictor.java	0.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #229      +/-   ##
============================================
+ Coverage     90.05%   90.25%   +0.20%     
- Complexity     2374     2389      +15     
============================================
  Files           135      140       +5     
  Lines          8901     8963      +62     
  Branches        865      860       -5     
============================================
+ Hits           8016     8090      +74     
+ Misses          462      455       -7     
+ Partials        423      418       -5     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
89.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud