Update relation logic to work for composite type as well

[xkello]

Description

Fixes #2315

The app treated every QGIS relation as an association, so deleting a parent never deleted its children. Children added while creating a new parent were also left behind as orphans when that parent was discarded. Relation strength is now read from the QGIS project, and composition relations are respected.

What changed

• InputUtils::deleteLinkedFeatures() deletes all features linked to a feature, including nested ones. Normally it only follows composition relations, but when a new feature is discarded it follows every relation.
• InputUtils::hasCompositionChildren() tells whether deleting features would also delete linked features.
• AttributeController::deleteFeature() and MultiEditManager::deleteSelectedFeatures() delete the linked features first and then continue with their original delete flow.
• AttributeController::hasCompositionChildren() and MultiEditManager::selectedFeaturesHaveCompositionChildren() drive the confirmation text.
• MMFormDeleteFeatureDialog.qml gets a deletesLinkedFeatures property that switches to the new description.
• MMFormPage.qml deletes everything linked to a new feature when it is discarded or deleted in add mode.

Behaviour

• Composition: deleting a parent also deletes its children, grandchildren and further levels.
• Association: unchanged, deleting a parent keeps its children.
• New parent discarded without saving: children created while adding it are deleted, for both strengths.
• Before deleting a parent with composition children, the dialog says the linked features will also be deleted. This works for one feature and for several selected features.

[github-actions]

Coverage Report for CI Build 39209144254

Coverage increased (+0.2%) to 60.019%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 192 coverage regressions across 4 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

192 previously-covered lines in 4 files lost coverage.

File	Lines Losing Coverage	Coverage
mm/app/attributes/attributecontroller.cpp	137	77.73%
mm/app/multieditmanager.cpp	46	54.61%
mm/core/merginapi.cpp	7	74.79%
mm/app/multieditmanager.h	2	0.0%

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Coverage Stats

Relevant Lines:	15940
Covered Lines:	9567
Line Coverage:	60.02%
Coverage Strength:	93.61 hits per line

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💛 - Coveralls

[github-actions]

📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	📬	Mergin Maps 73871 dmg	Expires: 04/01/2027	#7387
linux Build	📬	Mergin Maps 74131 x86_64	Expires: 04/01/2027	#7413
win64 Build	📬	Mergin Maps 65891 win64	Expires: 04/01/2027	#6589
Android Build	📬	Mergin Maps 869911 APK [armeabi-v7a]	Expires: 04/01/2027	#8699
	📬		Google Play Store	#8699
Android Build	📬	Mergin Maps 869951 APK [arm64-v8a]	Expires: 04/01/2027	#8699
	📬		Google Play Store	#8699
iOS Build	📬		Build number: 26.10.963911	#9639

[Withalion]

Will need some work, but looks alright. When you implement my comments request review from @tomasMizera

[github-actions]

📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	📬	Mergin Maps 74191 dmg	Expires: 17/01/2027	#7419
linux Build	📬	Mergin Maps 74451 x86_64	Expires: 17/01/2027	#7445
win64 Build	📬	Mergin Maps 66211 win64	Expires: 17/01/2027	#6621
Android Build	📬	Mergin Maps 873111 APK [armeabi-v7a]	Expires: 17/01/2027	#8731
	📬		Google Play Store	#8731
Android Build	📬	Mergin Maps 873151 APK [arm64-v8a]	Expires: 17/01/2027	#8731
	📬		Google Play Store	#8731
iOS Build	❌	Build failed or not found.		#9671

[github-actions]

📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	📬	Mergin Maps 74211 dmg	Expires: 17/01/2027	#7421
linux Build	📬	Mergin Maps 74471 x86_64	Expires: 17/01/2027	#7447
win64 Build	📬	Mergin Maps 66231 win64	Expires: 17/01/2027	#6623
Android Build	📬	Mergin Maps 873311 APK [armeabi-v7a]	Expires: 17/01/2027	#8733
	📬		Google Play Store	#8733
Android Build	📬	Mergin Maps 873351 APK [arm64-v8a]	Expires: 17/01/2027	#8733
	📬		Google Play Store	#8733
iOS Build	📬		Build number: 26.10.967311	#9673