#142 - Improves OpenAPI code generation (#144)

Addresses issues in OpenAPI code generation, specifically regarding:

# TLDR;
Fixes OpenAPI code generation by correctly handling `anyOf` schemas,
array types, various data types, and parameter handling, ensuring more
accurate and robust client code generation.

# Summary
This PR enhances the OpenAPI code generator to correctly handle various
OpenAPI schema constructs, including `anyOf` schemas, array types, and
different data types (integers, doubles, booleans). It also improves
parameter handling, including optional query parameters and header
parameters. The changes ensure that the generated client code is more
accurate and robust.

# Details
- Correctly maps `anyOf` schemas to nullable types in C#.
- Properly handles arrays of integers and references.
- Supports `int64` and `double` formats.
- Handles boolean types.
- Skips string enums.
- Correctly handles optional query parameters and header parameters.
- Sanitizes parameter names.
- Handles operations without `operationId`.
- Handles multiple response types.
- Handles empty response content.
- Handles required header parameters.
- Handles string default values.
- Handles operations with only error response.
- Handles required query parameters.
- Handles POST without request body.
- Handles boolean default value.
- Handles path with multiple operations.

Fixes #142

Author: MelbourneDeveloper