refactor: reduce duplication and increase test coverage

[ericodx]

Summary

• Extract recordResult in TestExecutionStage to eliminate duplicated result-handling logic between run and runSPM
• Add test for TestFilesHasher external symlink fallback path (line 20)

Type of Change

• refactor: A code change that neither fixes a bug nor adds a feature.
• test: Addition or correction of tests.

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?

• TestExecutionStage

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)
• All tests pass locally (swift test)