Fix conflicting type expectations in IndexSchema constructor and validation.
  The type annotation declared `fields: Dict[str, BaseField]` but the validator
  raised an error when fields was actually provided as a dict.

  Changes:
  - Update model validator to accept both list and dict formats for fields
  - Support dict with field definitions: {"field_name": {"name": "field_name", "type": "text"}}
  - Support dict with BaseField instances: {"field_name": BaseFieldInstance}
  - Maintain backward compatibility with existing list format
  - Use Field(default_factory=dict) instead of mutable default {}
  - Change fields type annotation from Dict to Mapping for better type safety
  - Use collections.abc.Mapping for isinstance checks
  - Use Sequence check with str/bytes exclusion for list detection
  - Add field name validation when BaseField is provided directly in dict format

Co-authored-by: Andrew Brookins <[email protected]>