Code cleanup: coverage improvements and dead code removal

[ericodx]

Summary

• Add LocalizedError conformance to BuildError and SimulatorError for proper error messages through generic catch blocks
• Collapse specific catch blocks into generic catch in SwiftMutationTesting.run, extract defaultLauncher helper
• Remove unreachable defensive code in IncompatibleMutantExecutor, ProjectDetector, MutationRewriter, SchemataGenerator, and DiscoveryPipeline
• Replace try! with guard let try? in MutantExecutor.rewriteForIncompatible
• Make killEscapedChildren internal for testability
• Extract ProcessRequest and ProcessRunner to reduce duplication across launchers
• Add ProjectDetector SPM test targets and testing framework detection
• Exclude Tests directory from SwiftLint and Periphery validation

Type of Change

• feat: A new feature has been added.
• refactor: A code change that neither fixes a bug nor adds a feature.
• test: Addition or correction of tests.
• chore: Other changes that do not fit into the previous categories.

Invariants Checklist

• Original project is never modified — all mutations happen inside an isolated sandbox
• xcodebuild build-for-testing runs exactly once for all schematizable mutants
• No mutant results are lost or duplicated
• Mutant positions (file, line, column) are accurate in all reported results
• A cancelled task never leaves a simulator slot permanently acquired from the pool
• schematizedContent never contains the __swiftMutationTestingID global declaration
• Swift 6 Strict Concurrency compatible
• Pipeline stages remain stateless pure transformations

Pipeline Impact

Which stages are affected?

• SandboxFactory
• BuildStage
• SimulatorPool
• TestExecutionStage
• IncompatibleMutantExecutor
• PerFileBuildFallback
• CacheStore
• Reporters (Text / JSON / HTML / Sonar)
• CLI / Configuration
• Models / RunnerInput contract
• None

Testing

• Unit tests added or updated
• Tests use mock ProcessLaunching — no real xcodebuild, xcrun simctl, or xcresulttool
• Tests use FileHelpers for any filesystem interaction (temp directories only)
• Snapshot tests added or updated (if reporter output format changed)
• Integration tests added or updated and tagged separately (if pipeline or CLI behavior changed)
• All tests pass locally (swift test)

Coverage

Line coverage improved from 99.10% (42 missed) to 99.53% (22 missed). Key additions:

• BuildErrorTests — errorDescription and equality
• SimulatorErrorTests — errorDescription for all cases
• MutantExecutorCoverageTests — catch block, errorLines fallback, removingCases default transition
• FallbackExecutorTests — SPM build success path
• KillEscapedChildrenTests — process matching with real process spawning
• defaultLauncher tests for xcode and spm project types