[test-improver] Add 29 Mockito unit tests for TaskService

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Goal & Rationale

TaskService is the largest and most bug-prone class in the codebase (~700 lines, multiple known bugs), yet it had zero pure unit tests. The existing TaskServiceTest uses @SpringBootTest (full context, H2 DB, DataInitializer seeding) — slow, fragile, and susceptible to ordering issues.

This PR adds 29 pure Mockito unit tests (@ExtendWith(MockitoExtension.class)) that run in under 1 second with no Spring context and no database. These tests are a prerequisite for the planned god-class refactoring tracked in Issue #16.

Approach

• @InjectMocks TaskService + @Mock for TaskRepository, UserRepository, ProjectRepository
• Covers: createTask, updateTask, getTask, deleteTask, assignTask, transitionStatus, getTaskStatistics, getAllTasks, getTasksByStatus, getTasksByAssignee
• 1 bug-pin test documenting the known division-by-zero in getTaskStatistics (Bug 🔴 [BUG] ArithmeticException: Division by zero on empty project dashboard #3)
• State machine tests for all documented transitionStatus transitions

Coverage Impact

No baseline JaCoCo data available on main branch yet (Issue #47). These tests exercise TaskService methods that were previously only covered through full-context @SpringBootTest.

Bug Pin: getTaskStatistics / Bug #3

// getTaskStatistics_noCompletedTasks_divisionByZeroBug
assertThrows(ArithmeticException.class,
    () -> taskService.getTaskStatistics(), "BUG #3: division by zero when no completed tasks");

This test documents the current broken behaviour. Do NOT modify the test to pass — fix getTaskStatistics() in the production code (guard completedCount == 0), then verify the other getTaskStatistics_withCompletedTasks_returnsCorrectCounts test still passes.

Trade-offs

• These are unit tests — they mock repositories. Integration behaviour (persistence, transaction boundaries) is not tested here. The existing TaskServiceTest @SpringBootTest tests continue to provide that coverage.
• sendTaskNotification is a private method that swallows all exceptions; not directly testable with Mockito.

Test Status

Tests run: 29, Failures: 0, Errors: 0, Skipped: 0
Time elapsed: 0.516 s
BUILD SUCCESS

Reproducibility

mvn test -Dtest=TaskServiceMockTest -Dmaven.repo.local=/tmp/m2 -B

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