Raise a clear error for schema DSL fields with no name

[code-with-rashid]

schema_dsl() crashes with an opaque IndexError when a field has no name before the colon (reachable from the CLI via llm schema dsl ':foo' and from resolve_schema_input):

>>> from llm.utils import schema_dsl
>>> schema_dsl(":just a description")
IndexError: list index out of range

After field.split(":", 1) the left-hand side is empty, so field_info.strip().split() returns [] and field_parts[0] is out of range.

This raises a ValueError naming the offending field instead, so the cause is obvious:

>>> schema_dsl(":just a description")
ValueError: Field is missing a name: ':just a description'

Valid schemas are unaffected.

Tests

Added test_schema_dsl_missing_field_name (parametrized over ':just a description' and 'name, :description') asserting the ValueError. pytest tests/test_utils.py passes (84 tests).

Closes #1466