refactor(tests): make string assertions and searches comparison-explicit (CA1307, CA1310)

[DeepDiver1975]

Adds an explicit StringComparison.Ordinal to all 51 flagged call sites (98 CA1307 + 6 CA1310 warnings). Every site is in a test project — production code was already comparison-explicit.

• 46 string assertions: xUnit Assert.Contains/StartsWith/EndsWith/DoesNotContain(string, string?) now pass StringComparison.Ordinal.
• 5 string search calls: string.Contains(string|char) / string.StartsWith(string) in test helpers get an explicit ordinal comparison.
• Chose Ordinal (deterministic, culture-free) over CurrentCulture — the matched strings are exact German UI/log fragments; the full suite still passes 801/1, confirming no assertion relied on culture-linguistic collapsing.
• Verification: CA1307 and CA1310 at 0 under AnalysisMode=All, zero errors, no change to any other rule; full suite 801 passed / 1 skipped.