Related Issues

• Resolves CATROID-1584: REFACTOR StitchAction to Kotlin (https://catrobat.atlassian.net/browse/CATROID-1584)
• Resolves CATROID-1585: REFACTOR tests of StitchAction to Kotlin(https://catrobat.atlassian.net/browse/CATROID-1585)

Description
This PR refactors the StitchAction class and its corresponding instrumented test StitchActionTest from Java to idiomatic Kotlin.

Key Changes

StitchAction.kt:

• Converted core logic to idiomatic Kotlin.
• Implemented Null Safety using safe calls (?.) and the Elvis operator (?:) to prevent NullPointerExceptions, fulfilling the requirement to eliminate !! assertions in production code.
• Updated property access for zIndex and embroideryThreadColor.

StitchActionTest.kt:

• Refactored the test suite to Kotlin.
• Optimized the mock setup to use lateinit var for the embroideryPatternManagerMock, allowing for cleaner verify calls without null-check clutter or !!.
• Corrected type handling for zIndex which is now safely handled as an Int.

General:

• Updated copyright headers to 2026.
• Verified that the project compiles and builds successfully.

Type of Change

• Refactoring (non-breaking change which improves code structure/readability)

How Has This Been Tested?

• Build Verification: Successfully ran ./gradlew assembleCatroidDebugAndroidTest to ensure all Kotlin components and test dependencies are correctly configured.
• Static Analysis: Verified that no !! assertions are used in the core logic and that null-safety patterns are correctly applied.

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