[Tests] Domain models: validation tests (#106)

[fboucher]

Replaces property-setter tests with meaningful validation tests for Post, PostL, Settings, and Summary domain models.

Changes

Post & PostL

• Test required field enforcement (PartitionKey, RowKey)
• Test nullable boolean states (is_read)
• Test default values (Word_count defaults to 0)
• Test empty string support where applicable

Settings

• Test ContainsPlaceholder validation attribute for SummaryPrompt and SearchPrompt
• Test required placeholder presence ({content}, {topic})
• Test null handling in validation
• Test required fields

Summary

• Test required fields (PartitionKey, RowKey)
• Test nullable field behavior
• Test string boolean values for IsGenerated

Impact

• Removed 8 trivial property-setter tests
• Added 22 validation and constraint tests
• Tests now document actual model invariants
• All 115 tests pass

Closes #106

[fboucher]

✅ APPROVED — Merge as-is

Review Summary

All acceptance criteria from issue #106 are fully met. This PR successfully transforms trivial property-setter tests into meaningful validation tests that document actual domain invariants.

Coverage Analysis

✅ Domain Model Validation Tests

• 22 new tests covering Post, PostL, Settings, and Summary
• Removed 8 trivial property-setter tests
• All tests are genuinely meaningful (not just renamed setters)

✅ ContainsPlaceholder Validation

• Settings.SummaryPrompt: Tests for required {content} placeholder (3 tests)
• Settings.SearchPrompt: Tests for required {topic} placeholder (3 tests)
• Uses proper validation framework with ValidationContext

✅ Required Field Enforcement

• Tests for PartitionKey/RowKey on all 4 models (8 tests total)
• Verifies C# required modifier enforcement

✅ Nullable Field Behavior

• Post/PostL is_read: Tri-state boolean tests (null/true/false)
• Summary nullable fields: IsGenerated, FileName, PublishedURL
• Theory-based parameterized tests where appropriate

✅ Default Values

• Post.Word_count defaults to 0 ✓
• Nullable booleans default to null ✓

✅ CI Status

All 115 tests pass. Both CI checks successful:

• .NET Tests: ✓
• Run Unit Tests: ✓

Code Quality

• Clear, descriptive test names
• Proper use of FluentAssertions
• Comprehensive edge case coverage
• Well-structured AAA pattern

No gaps identified. Ready to merge.

---

Review by Livingston (Code Review Specialist)
Full report: .squad/decisions/inbox/livingston-pr108-review.md

[github-actions]

Package	Line Rate	Branch Rate	Health
NoteBookmark.AIServices	97%	64%	✔
NoteBookmark.Domain	5%	9%	❌
NoteBookmark.Api	91%	80%	✔
NoteBookmark.ServiceDefaults	96%	75%	✔
NoteBookmark.Domain	83%	72%	✔
NoteBookmark.AIServices	97%	64%	✔
NoteBookmark.Domain	5%	9%	❌
Summary	55% (919 / 1435)	39% (193 / 355)	❌