refactor: reduce complexity in model_generator.type_converter (fixes #98)

## Summary
Refactored `type_converter` function to eliminate high cyclomatic complexity,
code duplication, and fragile import aggregation. The function is now more
maintainable, testable, and easier to extend with future OpenAPI 3.1 features.

## Changes

### Extracted Helper Functions
- `_normalize_schema_type()`: Unified handling of DataType enum, strings, and lists
- `_is_type()`: Simplified type checking across all schema type representations
- `_handle_format_conversions()`: Consolidated UUID/datetime logic (eliminated 3x duplication)
- `_wrap_optional()`: Replaced scattered pre_type/post_type pattern
- `_collect_unique_imports()`: Safe import aggregation without blind indexing
- `_convert_primitive_type()`: Handles string, int, float, bool, object, null types
- `_convert_array_type()`: Isolated array type handling with items processing
- `_convert_composite_schema()`: Unified allOf/oneOf/anyOf composition logic

### Main Function Improvements
- Reduced from ~270 lines to 53 lines (80% reduction)
- Cyclomatic complexity reduced from >16 to <10
- Removed noqa C901 suppression comment
- Clear separation of concerns with early returns

## Benefits
- **Maintainability**: Each responsibility isolated in well-documented helpers
- **Safety**: No more direct indexing of potentially empty import lists
- **Testability**: Individual functions can be tested independently
- **Extensibility**: Adding new types/formats now straightforward
- **Zero behavioral changes**: All 250 tests pass (55 model_generator specific)

## Metrics
| Metric                    | Before | After | Improvement        |
|---------------------------|--------|-------|--------------------|
| Function size             | ~270   | 53    | 80% reduction      |
| Cyclomatic complexity     | >16    | <10   | Below threshold    |
| Code duplication (UUID)   | 3x     | 1x    | 100% eliminated    |
| Import aggregation safety | Fragile| Safe  | No IndexError risk |

Closes #98

Author: Doug Borg