fix: review nullability annotations around SolutionManager

[triceo]

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[sonarqubecloud]

Quality Gate failed

Failed conditions
65.2% Coverage on New Code (required ≥ 70%)

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[Copilot]

Pull Request Overview

This PR fixes nullability annotations around the SolutionManager by migrating from @NonNull to @NullMarked annotation strategy and correcting parameter nullability declarations.

• Replaces @NonNull annotations with @NullMarked class-level annotations for better null safety defaults
• Corrects proposition function parameter types to allow nullable return values in recommendation APIs
• Fixes equals/toString implementations to handle null values properly in MutableReference

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
MutableReference.java	Adds null safety annotations and fixes equals/toString to handle null values
RecommendationConstructor.java	Updates interface to use @NullMarked and allow nullable result parameter
DefaultSolutionManager.java	Migrates from @nonnull to @NullMarked and fixes score comparison logic
DefaultRecommendedFit.java	Updates record to use @NullMarked instead of @nonnull
DefaultRecommendedAssignment.java	Updates record to use @NullMarked instead of @nonnull
AssignmentProcessor.java	Adds null safety and fixes resource management with try-with-resources
Assigner.java	Updates to @NullMarked and adds null check for lookUpWorkingObject result
ShadowVariableUpdateHelper.java	Adds @NullMarked and improves null safety with Objects.requireNonNull
SolutionManager.java	Interface migration from @nonnull to @NullMarked with corrected parameter types
RecommendedFit.java	Adds @NullMarked annotation
RecommendedAssignment.java	Updates interface to use @NullMarked and fixes comment typo

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