Add 354 unit tests and calibrate mutation testing thresholds toward 80%

[Copilot]

Baseline mutation score is 37.83% — low by industry standards (80%+ is good). This PR adds comprehensive test coverage for previously untested code and recalibrates Stryker thresholds to drive improvement.

Threshold changes

Updated stryker-config.json from break=35/low=40/high=60 to:

"thresholds": {
  "high": 80,
  "low": 50,
  "break": 37
}

• break=37: At baseline — any regression fails CI immediately
• low=50: Mid-term target
• high=80: Industry-standard good score

Documentation

Added honest score assessment to docs/mutation-testing.md with industry benchmark ranges and root cause analysis (596 no-coverage + 707 survived mutants in console/pipeline/IO code).

New tests (354 tests across 48 files)

Coverage added for previously untested subsystems:

• Utilities & validators — PascalCaseHumanizer, StringExtensions, Timer, Constants, ExitCode, VariableNameValidator, FunctionNameValidator, HeaderNameValidator
• HTTP parsing — MethodAndUriParser, HeaderParser, BodyParser, CommentLineParser, EmptyLineParser, HttpParsingContext
• Header handlers — All 10 implementations (Default, Authorization, ContentType, Host, UserAgent, Date, Connection, ContentDisposition, ContentEncoding, ContentLanguage)
• Body resolvers — JsonBodyResolver, XmlBodyResolver
• Builder patterns — ApplicationContextOptionsBuilder, BaseDirectivePatternBuilder, HttpDirectivePatternBuilder
• Models & records — TestResultsSummary, CollectionTestResultsSummary, File, Folder, InternalFile, ExternalFile, Script, TestCase, ScriptReference, TestDescription, RequestVariableDescription
• Registries & structure — ExternalFilesRegistry, CollectionStructure, NoAuthProvider
• Directives & constants — AuthConstants, AuthDirectives, RetryingConstants, RetryingDirectives, TestDirectives
• Path resolution — PathProvider, RelativePathResolver, PathExtensions
• Contexts — RequestExecutionContext, ApplicationContextOptions, HttpParsingContext
• Retry parsers — RetryStrategyDirectiveLineParser, RetryUntilStatusCodesDirectiveLineParser, RetryExplicitPropertiesDirectiveLineParser
• Functions & JSON — Function<T> variants, DefaultFunctionsRegistrator, CaseInsensitiveExpandoObject, Environment

Test count: 449 → 803 passing. The 1 pre-existing failure (AuthHttpMessageHandlerShould) is unchanged.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• example.com
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/TeaPie.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/TeaPie.Tests.deps.json /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/testhost.dll --port 34997 --endpoint 127.0.0.1:034997 --role client --parentprocessid 4326 --telemetryoptedin false (dns block)
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/TeaPie.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/TeaPie.Tests.deps.json /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/testhost.dll --port 33035 --endpoint 127.0.0.1:033035 --role client --parentprocessid 5388 --telemetryoptedin false (dns block)
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/TeaPie.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/TeaPie.Tests.deps.json /home/REDACTED/work/TeaPie/TeaPie/tests/TeaPie.Tests/bin/Debug/net8.0/testhost.dll --port 39445 --endpoint 127.0.0.1:039445 --role client --parentprocessid 7310 --telemetryoptedin false (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

---

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[Burgyn]

Code review

Found 1 issue:

1. ContainSingle("value1") uses the wrong FluentAssertions overload -- the string parameter is treated as a "because" failure message, not the expected value. This assertion only verifies the collection has exactly one element but does not verify the element is "value1". The test passes regardless of what value the header contains. Fix: use .ContainSingle().Which.Should().Be("value1") or .ContainSingle(x => x == "value1").

TeaPie/tests/TeaPie.Tests/Http/Headers/DefaultHeaderHandlerShould.cs

Lines 14 to 19 in e25f02b

	_handler.SetHeader("value1", request);
	request.Headers.GetValues("X-Custom").Should().ContainSingle("value1");
	}

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