[test-improver] Add model unit tests: User.getDisplayName() logic and Task.toString() NPE bugs

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Goal & Rationale

Two model classes had untested logic with real bugs:

1. User.getDisplayName() — a 3-branch method with surprising partial-name behaviour: if firstName is set but lastName is null (or vice versa), the set name is silently discarded and the method falls back to full_name/username. A caller who sets firstName but forgets lastName gets no error; their data is lost.

2. Task.toString() — two documented NPEs: assignee_id.toString() (line 122) and due_date.trim() (line 123) both dereference fields without null checks. Any Task constructed via the no-arg JPA constructor will throw when toString() is called (logging, debugger, etc.).

Approach

Pure no-Spring JUnit 5 tests (@ExtendWith(MockitoExtension.class) not even needed — plain POJO construction). Runs in < 1 s with no context startup.

UserModelTest (10 tests):

Test	Branch
getDisplayName_bothFirstAndLastName_returnsCombined	First+Last → "John Smith"
getDisplayName_noFirstOrLastName_usesFullName	full_name fallback
getDisplayName_noNamesAtAll_usesUsername	username fallback
getDisplayName_firstNameSetButLastNameNull_fallsBackToFullName	Bug pin
getDisplayName_firstNameSetLastNameNullNoFullName_fallsBackToUsername	Bug pin
getDisplayName_lastNameSetButFirstNameNull_fallsBackToUsername	Bug pin
getDisplayName_firstAndLastNamePrecedeFullName	Precedence
getDisplayName_fullNamePrecedesUsername	Precedence
getDisplayName_emptyFirstAndLastName_returnsSpaceSeparated	Edge case
getDisplayName_defaultConstructor_noNPE	Null safety

TaskModelTest (7 tests):

Test	Branch
shortConstructor_setsFieldsCorrectly	Short constructor
defaultConstructor_allFieldsDefaultToNull	Default constructor
toString_nullAssigneeId_throwsNPE	Bug pin
toString_nullDueDate_throwsNPE	Bug pin
toString_defaultConstructorFields_throwsNPE	Bug pin
toString_allRequiredFieldsSet_noException	Happy path
attachmentUrls_initializedAsEmptyList	Initial state

Coverage Impact

Tests are pure POJO — no Spring context, no DB. Adds coverage for User.getDisplayName() (all 3 branches) and Task.toString() (NPE paths).

Trade-offs

• Bug-pin tests intentionally assert the current buggy behaviour (assertThrows(NPE)) so that fixing the bug requires updating the tests — making the fix visible.
• No Spring context → no DataInitializer race conditions.

Reproducibility

mvn test -Dtest="UserModelTest,TaskModelTest" -Dmaven.repo.local=/tmp/.m2 -B

Test Status

✅ 17/17 tests pass (10 UserModelTest + 7 TaskModelTest)
✅ Existing test suite unaffected (pre-existing failures on testGetQuarter and testGetTaskStatistics are unchanged)

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