Fix schema DSL crash on empty field names before :

[TheBestVivBoy]

Summary

Fix schema_dsl() so inputs like :foo or name, :foo raise a validation error instead of crashing with IndexError.

Root cause

When a schema DSL field contains a colon but no field name on the left side, field_info.strip().split() returns an empty list. The existing code indexed field_parts[0], which raised IndexError.

Changes

• validate that each schema DSL field has a name before :
• return a clean CLI error for llm schemas dsl
• return a clean validation error when schema DSL is resolved via --schema
• add regression tests for the utility and CLI paths

Validation

• uv run pytest tests/test_utils.py -k schema_dsl
• uv run pytest tests/test_llm.py -k 'schemas_dsl or schema_using_dsl_invalid_empty_field_name'
• uv run black llm/utils.py llm/cli.py tests/test_utils.py tests/test_llm.py
• uv run ruff check llm/utils.py llm/cli.py tests/test_utils.py tests/test_llm.py
• uv run pytest tests/test_utils.py tests/test_llm.py
• uv run llm schemas dsl ':foo'
• uv run llm schemas dsl 'name:text'