[test-improver] Add 27 unit tests for UserService using Mockito

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Goal & Rationale

UserService had ~1% code coverage — the entire authentication and user-management service was effectively untested. This is the highest-risk service in the codebase: it handles login, password reset, and user creation.

This PR adds 27 pure Mockito unit tests covering all public methods, with no Spring context or database — tests run in under 1 second.

Approach

Used @ExtendWith(MockitoExtension.class) with a mocked UserRepository. Each test is isolated — no shared state between tests.

Tests Added (27)

Area	Tests
createUser	success, duplicate username, duplicate email
authenticate	valid credentials, unknown user, inactive account, wrong password, failed-attempt increment
getUser	found, not found
updateUser	not found, null fields preserved, role escalation bug pin
requestPasswordReset	unknown email, token returned + persisted
resetPassword	invalid token, token cleared + password updated
searchUsers	blank keyword → empty, null keyword → empty, valid keyword delegates
getUserDisplayName	not found, first+last, full_name fallback, username fallback
getActiveUsersByRole	filters inactive and deleted users
deactivateUsers	returns count, skips non-existent users

Bug Pin: Role Escalation (Security)

updateUser_roleEscalationBugPin_anyUserCanBecomeAdmin documents that updateUser() accepts a role field change with no authorization check — any caller can set their own role to "admin". This test pins the current behaviour so a future fix is visible in the test suite.

Coverage Impact

Before this PR, UserService had ~1% instruction coverage (Spring Boot startup touches some paths). After:

Metric	UserService Before	UserService After (est.)
Instructions	~1%	~80%+
Branches	~0%	~80%+
Methods	0/11	~10/11

Exact numbers depend on whether JaCoCo PR #47 is merged; the estimate is based on reading all branches.

Trade-offs

• Tests use Mockito, matching the pattern established in the controller @WebMvcTest PRs
• No @SpringBootTest overhead — each test class runs in isolation under 1 second
• Role escalation bug pin will need updating once the bug is fixed

Reproducibility

mvn test -Dtest=UserServiceTest -B
# All 27 tests should pass in ~1s
mvn test -B
# Full suite: 65 tests, 1 failure, 1 error (both pre-existing, not caused by this PR)

Test Status

Tests run: 27, Failures: 0, Errors: 0, Skipped: 0

Pre-existing failures (not caused by this PR):

• DateUtilsTest.testGetQuarter — Bug 🔴 [BUG] getQuarter() returns 0-3 instead of 1-4 (off-by-one bug) #6 (off-by-one)
• TaskServiceTest.testGetTaskStatistics — Bug 🔴 [BUG] ArithmeticException: Division by zero on empty project dashboard #3 (division by zero)

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