Add JaCoCo coverage enforcement and comprehensive unit tests (100% line coverage)

[devin-ai-integration]

Summary

Adds JaCoCo Maven plugin with enforced coverage gates (≥95% line, ≥85% branch) and comprehensive unit tests to achieve 100% line coverage and 98.3% branch coverage.

Coverage Before → After

Metric	Before	After
Line Coverage	~70.7% (108/369 lines missed)	100% (0/369 lines missed)
Branch Coverage	~0% (no JaCoCo)	98.3% (1/58 branches missed)
Total Tests	63	~105

Changes

• pom.xml: Added JaCoCo plugin (v0.8.11) with prepare-agent, report, and check executions enforcing 95% line / 85% branch coverage
• TodoViewControllerTest (new, 15 tests): All view endpoints — list with filters/search/pagination, create/edit forms, create/update/delete/toggle actions, validation error handling
• DataInitializerTest (new, 2 tests): Seed logic when DB is empty vs. already populated
• TodoTest (new, 8 tests): Entity lifecycle callbacks (onCreate/onUpdate), completedAt behavior, getters/setters, toString
• TodoResponseTest (new, 7 tests): fromEntity() mapping with all date permutations, overdue logic for each status
• TodoRequestTest (new, 3 tests): Both constructors and setters
• GlobalExceptionHandlerTest (new, 3 tests): All three exception handlers tested directly
• TodoNotFoundExceptionTest (new, 1 test): Exception message and ID storage
• PriorityTest / TodoStatusTest (new, 2 tests each): Enum display names and values
• TodoApplicationTest (new, 1 test): Main method with mocked SpringApplication.run
• TodoServiceTest (extended, +5 tests): getTodosByStatus, getTodosByPriority, empty due date edge cases

Review & Testing Checklist for Human

• Run ./mvnw verify locally to confirm all tests pass and JaCoCo check enforces the 95%/85% gates
• Spot-check that new tests exercise real behavior (not just covering lines trivially) — particularly TodoViewControllerTest and DataInitializerTest
• Verify no production code was changed solely to satisfy coverage

Notes

• The JaCoCo check runs during mvn verify and will fail the build if coverage drops below thresholds
• Coverage reports are generated at target/site/jacoco/index.html
• All new tests are pure unit tests — no real network, DB, or filesystem access

Link to Devin session: https://app.devin.ai/sessions/421c50f6e096408c88800bcfd6bce490
Requested by: @clivingston-cognition

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.