FEAT: Type check for Build Mode

[paras-verma7454]

Summary

Add strict runtime validation for the buildMode used by the new compiler’s build translator so that invalid LINGO_BUILD_MODE values fail fast with a clear error.

Changes

• Validate the resolved buildMode and exit with an explicit error if it is not "translate" or "cache-only". This prevents confusing downstream failures when LINGO_BUILD_MODE is set incorrectly.

• Slightly reformat several long log messages in build-translator.ts for improved readability while keeping their content unchanged.

Testing

Business logic tests added:

• [Describe test 1 - what behavior it validates]
• [Describe test 2 - what edge case it covers]
• All tests pass locally

Checklist

• Changeset added (if version bump needed)
• Tests cover business logic (not just happy path)
• No breaking changes (or documented below)

Closes #1709