Handle invalid collection getKey return values

[KyleAMathews]

Throws InvalidKeyError when getKey returns values other than string or number (e.g., null, objects, booleans). The validation is optimized to only require 1 typeof check on the happy path (string keys).

• Add InvalidKeyError class to errors.ts
• Update generateGlobalKey to validate key type before generating key
• Add tests for invalid key types (null, object, boolean)
• Add tests confirming valid keys (string, number, empty string, zero)

🎯 Changes

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).