Convert single-character StrCat string args to char literals

[Copilot]

StrCat calls were using single-character string literals in many places (e.g. ">"-style delimiters and punctuation), which is inconsistent with the intended char-based usage for single codepoints. This updates those call sites to pass character literals instead.

• Scope

  • Replaced single-character string-literal arguments in StrCat(...) with equivalent char literals.
  • Applied consistently across converter paths that build emitted Rust text.

• Files updated

  • cpp2rust/converter/converter.cpp
  • cpp2rust/converter/models/converter_refcount.cpp
  • cpp2rust/converter/plugins/emplace_back.cpp

• Change pattern

  • Preserved call structure and ordering; only argument literal types changed where the argument is a single character.
  • Multi-character strings remain unchanged.

// before
StrCat(".to_string_iterator() as StringIterator<",
       ToString(loop_var->getType().getNonReferenceType()), ">");

// after
StrCat(".to_string_iterator() as StringIterator<",
       ToString(loop_var->getType().getNonReferenceType()), '>');