[test-improver] test: add StringComparison overload and null-handling tests for Assert.StartsWith/EndsWith

[Evangelink]

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Goal and Rationale

Assert.StartsWith, Assert.DoesNotStartWith, Assert.EndsWith, and Assert.DoesNotEndWith each have two overloads: one defaulting to StringComparison.Ordinal and one accepting an explicit StringComparison. The existing tests in AssertTests.StartsWithEndsWith.cs only exercised the default Ordinal path — the StringComparison overloads and null parameter handling were completely untested.

These are real logic paths worth testing:

• The StringComparison overload is the core of case-insensitive string assertion, commonly used in multi-culture test suites.
• Null handling follows CheckParameterNotNull which throws a specific AssertFailedException message — a contract worth verifying.

Approach

Added 18 new tests to AssertTests.StartsWithEndsWith.cs covering:

Category	Count	Methods
OrdinalIgnoreCase success (case mismatch / exact match passes)	6	StartsWith × 2, DoesNotStartWith × 1, EndsWith × 2, DoesNotEndWith × 1
OrdinalIgnoreCase failure message includes comparison:	4	all four methods
Null value → AssertFailedException	4	all four methods
Null prefix/suffix → AssertFailedException	4	all four methods

Coverage Impact

File	Before	After
AssertTests.StartsWithEndsWith.cs	8 tests	26 tests (+18)
Assert.StartsWith.cs / Assert.EndsWith.cs	StringComparison overloads untested	now covered

Trade-offs

• Tests are straightforward string comparisons — low maintenance burden.
• Null-handling tests depend on the exact exception message format; if the format changes, these tests will catch the regression intentionally.

Reproducibility

./build.sh --projects "$(pwd)/test/UnitTests/TestFramework.UnitTests/TestFramework.UnitTests.csproj" -test

Test Status

✅ All TestFramework.UnitTests tests pass (net8.0 and net9.0)

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[Copilot]

Pull request overview

Adds missing unit-test coverage for the StringComparison overloads (and null-parameter behavior) of MSTest’s string boundary assertions in src/TestFramework, ensuring structured failure messages and CheckParameterNotNull contracts are exercised.

Changes:

• Added OrdinalIgnoreCase coverage for Assert.StartsWith / Assert.EndsWith success and for failure messages including the comparison: evidence line.
• Added null-parameter tests for value and prefix/suffix parameters across StartsWith, DoesNotStartWith, EndsWith, and DoesNotEndWith.
• Validated expected/actual payload values for new structured-failure cases.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.StartsWithEndsWith.cs	Adds tests covering StringComparison overload behavior and null-parameter exception contracts for StartsWith/EndsWith assertions.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1