fix(schema): log warning when removing non-existent field

[lotaar]

📝 Summary

Adds a warning when IndexSchema.removeField() is called with a field name that does not exist in the schema. The warning includes the requested field name and a list of currently available fields to help diagnose typos or incorrect API usage.

🎯 Motivation

Previously, removeField() silently no-opped when the target field was missing. This made it difficult to catch configuration errors or typos during development, as invalid removal attempts went completely unnoticed.

🛠 Changes

• src/schema/schema.ts: Updated removeField() to emit a console.warn() when the field is not found, matching the existing warning pattern used in src/query/vector.ts.
• tests/unit/schema/schema.test.ts: Added 3 unit tests:
  • ✅ Warning includes field name and available fields when schema is populated
  • ✅ Warning shows (none) when schema is empty
  • ✅ No warning is emitted when an existing field is successfully removed (regression guard)

🧪 Testing

• npm run test:unit passes
• npm run type-check & npm run type-check:tests pass
• npm run lint & npm run format:check pass
• Tests follow the project's TDD workflow and use vi.spyOn(console, 'warn') consistent with existing test patterns.

✅ Acceptance Criteria

• Warns when the field isn't present in the schema
• Includes the field name and list of known field names in the warning
• Unit tests added

🔗 Closes #13