[nevridge]

Summary

Removes the FluentAssertions dependency (Xceed Community License, not allow-listed in dependency-review.yml) and rewrites the test assertions to xunit's built-in Assert.*. Docs and agent instructions that described FluentAssertions are updated too.

Closes #294

What changed

• TaskFlow.Api.Tests.csproj: removed the FluentAssertions 8.10.0 package reference.
• All 36 test files under TaskFlow.Api.Tests/: removed using FluentAssertions; and rewrote every .Should() call (863 lines) to Assert.*. No tests were added, removed, skipped or restructured.
• Docs and agent files: README.md, TaskFlow.Api.Tests/README.md, docs/CONTRIBUTING.md, docs/SERVICE_REGISTRATION_PATTERN.md, .claude/agents/backend-builder.md, .claude/agents/test-verifier.md now describe xunit Assert.*.
• No production code, generated client or frontend changes. .github/dependabot.yml already had no FluentAssertions ignore entry.

Semantic-preservation decisions

• BeEquivalentTo on collections: Assert.Equal(expected.Order(), actual.Order()) or Assert.Single, so extra or missing items still fail. Validation-failure lists use Assert.Equivalent(..., strict: true).
• TaskRepositoryTests entity BeEquivalentTo: Assert.Same(result, savedTask), since FindAsync returns the tracked instance (at least as strict, avoids cyclic graph walking).
• ContainInOrder: filter to the expected items, then Assert.Equal (subsequence in order).
• WithMessage("*x*"): Assert.Throws<T> then Assert.Contains(..., StringComparison.OrdinalIgnoreCase) to match the old case-insensitive wildcard.
• NotThrow / NotThrowAsync: Assert.Null(Record.Exception(...)) / Record.ExceptionAsync.
• BeCloseTo: Assert.Equal(DateTime.UtcNow, actual, TimeSpan.FromSeconds(5)), with Assert.NotNull first for nullable values.
• RouteValues: TryGetValue plus Assert.Equal(<int>, value) (boxed ints confirmed by passing tests).
• "Because" messages kept: xunit message overload where one exists, otherwise a // comment directly above the assertion.

Test results

• Baseline (unchanged code): 417 passed, 0 skipped; 91.6% line coverage with the ci.yml flags and excludes (42.57% with plain --coverlet, which has no excludes and does not meet the gate on its own).
• After: 417 passed, 0 skipped; 93.08% line coverage with the ci.yml flags, gate threshold 75.
• [Fact]/[Theory] attributes (383) and data rows (54) are identical to main.
• dotnet format TaskFlow.sln --verify-no-changes: clean.
• rg for FluentAssertions / .Should() across code and the six docs/agent files: no matches.
• Mutation checks (extra collection item, wrong order, wrong message substring, thrown exception in a not-throw test) each made the matching test fail; mutations reverted.
• Independent test-verifier and implementation-validator runs: 0 Critical, 0 Important.

Residual findings (cosmetic, no behavior change)

• A few redundant ! after Assert.NotNull remain.
• About 15 redundant parentheses around awaited expressions, e.g. Assert.Equal(0, (await ...)).
• A few .Count() calls on IEnumerable<T> locals.

Follow-up

• chore(deps): Bump FluentAssertions and 10 others #293 (Dependabot NuGet group PR) should be rebased onto this change; the Review Dependencies for Vulnerabilities check should then pass (AC10, not verifiable until then).