fix: improve file type detection and C/C++ AST extraction

[yangxinxin-7]

• Fix ResourceContentType.UNKNOWN AttributeError by returning None for
  unrecognized file types, with WARNING log and summary fallback
• Expand text_extensions with lua, common scripting, config, and more
  programming language suffixes (.dart, .vue, .proto, .tf, etc.)
• Enhance CppExtractor to handle typedef struct and function prototypes,
  refactor with shared _process_siblings helper for top-level and namespace
• Add unit tests for typedef struct, function prototypes, namespace, and
  declaration+definition coexistence scenarios