fix: add assertion analyzers for common mistakes (prefer IsNull, IsTrue/IsFalse)

[thomhurst]

Closes #4874. Adds TUnitAssertions0014 and TUnitAssertions0015. 56/56 tests passing.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

The two new Roslyn analyzers (PreferIsNullAnalyzer / TUnitAssertions0014 and PreferIsTrueOrIsFalseAnalyzer / TUnitAssertions0015) are well-implemented:

• Correct use of Roslyn Operations API — Using IInvocationOperation and related operation types is more robust than syntax-based analysis, correctly handling implicit conversions, default values, and constant folding.
• Argument lookup by parameter name (a.Parameter?.Name == "expected") correctly handles extension method receivers vs. regular arguments.
• Null detection covers the important cases: direct null literals, constant values that are null, IConversionOperation wrapping null, and IDefaultValueOperation on reference types.
• Boolean detection similarly covers literals, constant values, and conversions.
• Diagnostic message format is consistent — TUnitAssertions0014 has no placeholders, TUnitAssertions0015 correctly passes suggestedMethod and literalText matching the {0}/{1} placeholders in the resource string.
• Tests cover both positive (diagnostic raised) and negative (no diagnostic) cases for all relevant scenarios.
• CLAUDE.md rules — All critical rules are either explicitly exempted for analyzer changes (Dual-Mode, Snapshot Testing) or are not implicated (VSTest, AOT, async blocking).