OpenSystemsDevelopment / qpdf

- Add null pointer checks for schema objects.
- Refactor `checkSchemaInternal` for better readability and maintainability.
- Replace redundant error construction logic with lambda-based approach.
- Simplify array and dictionary schema handling.
- Remove unused test coverage statistics.

…std::string_view`, improving performance and code clarity.

Converted multiple occurrences of `count()` to `contains()` throughout the codebase where the goal was to check key existence in containers. This improves code readability and aligns with modern C++ practices, particularly with C++20, making the intent more explicit and potentially aiding performance.

Replace `count` method with iterator-based lookup to improve readability and reduce redundancy. Simplify conditional logic for determining the encryption filter, ensuring the default case remains clear.

Replaced manual key-value access with structured bindings in loops for better readability and modern C++ practices. Simplified error handling using `emplace_back` instead of `push_back`. Improved consistency and clarity in schema validation logic.

Add new private Util.hh header to define inline functions and expose as
ordinary functions in QUtil.

Remove member encoded and encode strings only when required.

Encoded is only used when writing JSON. Since most JSON writing is now
done on the fly without creating a JSON representation, creating and
storing the encoded string in almost all cases no longer serves any
purpose.

If duplicate keys are encountered, overwrite earlier values with the latest
value.

Create a simple utility class for writing JSON to a pipeline.

Using search and replace.

Recognise that JSON objects are effectively shared pointers to JSON_value.

Eliminate the use of shared pointers in JSONParser

Reduce boilerplate and increase efficiency by avoiding setting and
branching on action and ready in getToken.

Also, reject them in strings.

Handle all incomplete token type errors in tokenError.

(except '\\' and '\uXXXX')