AST builder: validate and harden qualified type parsing

[YasiruRF]

Problem

Qualified type parsing in visit_QualifiedType splits on :: without explicit validation for malformed names.

Proposed change

• Add validation for empty package/name and malformed separators.
• Emit diagnostics/logging for invalid qualified types.
• Keep graceful fallback behavior where appropriate.

Acceptance criteria

• Malformed qualified types produce clear diagnostics.
• Well-formed types continue to parse unchanged.
• New tests cover edge cases (::Type, pkg::, a::b::c policy).