Stabilize timeout partial output test

[maniramezan]

Summary

• make timeoutPreservesPartialOutput wait for a marker after stdout is emitted
• assert the deterministic partial stdout value instead of racing timeout capture

Validation

• swift-format lint --strict Tests/SwiftyShellTests/Core/CommandTests.swift
• swift test --filter timeoutPreservesPartialOutput -Xswiftc -warnings-as-errors
• swift test -Xswiftc -warnings-as-errors
• swift -warnings-as-errors Scripts/validate-traits.swift
• swift -warnings-as-errors Scripts/validate-docc-coverage.swift
• swift package -Xswiftc -warnings-as-errors --allow-writing-to-directory docs generate-documentation --target SwiftyShell --output-path docs --transform-for-static-hosting --hosting-base-path SwiftyShell
• swift test --enable-all-traits --enable-code-coverage -Xswiftc -warnings-as-errors
• swift -warnings-as-errors Scripts/validate-code-coverage.swift --input .build/arm64-apple-macosx/debug/codecov/SwiftyShell.json --minimum-line-coverage 84