Complete tests for all `using` declaration forms

[alandefreitas]

Problem

Our test suite covers only a small subset of using declarations. We miss cases such as aliases, inheriting constructors, using-enum, using directives, and namespace-level imports. A more complete list of examples is available here: https://en.cppreference.com/w/cpp/language/using_declaration.html

Proposed solution

• Build a test matrix from the cppreference list of using forms.
• Add coverage for each scenario, including class shadows and options that import members into the current namespace.
• Confirm that the extractor handles all forms consistently.

Requirements

• Test suite exercises every using declaration enumerated on cppreference.
• Edge cases (e.g., friend declarations that use using) are validated.
• Documentation references the coverage table.