[codex] Pin dependencies and guard copied samples

[rudrankriyam]

Summary

• Pins the direct FoundationModelsKit package references to the currently resolved commit instead of tracking main.
• Pins the direct Xcode project references for FoundationModelsKit and LumoKit to their currently resolved commits.
• Adds a PR workflow that runs swift test and typechecks the copyable BookPlaygrounds chapter snippets.
• Fixes the copied generation-options sample/template to use the CI-compatible GenerationOptions(sampling:) label.

Notes

The remaining branch: main entries in the Xcode Package.resolved file are transitive requirements from the LumoKit dependency graph (swift-embeddings and VecturaKit). I left those alone because hand-editing generated lockfile state would be reverted by Xcode resolution unless the upstream manifests change or this repo takes explicit ownership of those transitive pins.

Verification

• swift test
• bash Tools/BookPlaygrounds/typecheck.sh
• xcodebuild -quiet -project FoundationLab.xcodeproj -scheme 'Foundation Lab' -destination 'generic/platform=macOS' CODE_SIGNING_ALLOWED=NO build
• git diff --check