Add unit tests for EnvironmentFilesParser utility

[Copilot]

EnvironmentFilesParser contains non-trivial parsing logic for environment config files (log levels, GC params, mono debug, HTTP handlers, assembly preload, broken exception transitions) with no test coverage.

Tests added

• Flag detection: MONO_LOG_LEVEL=, MONO_GC_PARAMS=, MONO_DEBUG=, XA_HTTP_CLIENT_HANDLER_TYPE=
• XA2000 warning: MONO_GC_PARAMS=bridge-implementation=old triggers coded warning
• Sequence points: MONO_DEBUG= appends gen-compact-seq-points when SequencePointsMode != None, skips if already present
• Line filtering: mono.enable_assembly_preload= and XA_BROKEN_EXCEPTION_TRANSITIONS= set flags but are excluded from EnvironmentVariableLines
• AreBrokenExceptionTransitionsEnabled: true/false paths
• Edge cases: null environments, multi-file flag accumulation

[Test]
public void MonoGCParams_OldBridgeWarning ()
{
	var envFile = CreateEnvFile ("MONO_GC_PARAMS=bridge-implementation=old");
	var warnings = new List<BuildWarningEventArgs> ();
	var engine = new MockBuildEngine (TestContext.Out, warnings: warnings);
	var log = new TaskLoggingHelper (engine, "Test");
	var parser = new EnvironmentFilesParser ();

	parser.Parse (new [] { envFile }, SequencePointsMode.None, log);

	Assert.IsTrue (parser.HaveMonoGCParams);
	Assert.AreEqual (1, warnings.Count);
	Assert.AreEqual ("XA2000", warnings [0].Code);
}

[Copilot]

Pull request overview

Adds a new NUnit test fixture covering the previously untested EnvironmentFilesParser utility, exercising flag detection, the XA2000 warning, sequence-points appending, line filtering, AreBrokenExceptionTransitionsEnabled, and basic edge cases.

Changes:

• New EnvironmentFilesParserTests fixture with per-test temp-file setup/teardown
• Tests for each parser flag (HaveLogLevel, HaveMonoGCParams, HaveMonoDebug, HaveHttpMessageHandler, BrokenExceptionTransitions)
• Tests for XA2000 warning, MONO_DEBUG sequence-points append/dedup, multi-file accumulation, and null input