Commit 3: Add test.cpp corpus wrapped in std::regex stubs; generate .expected

Introduces cpp/ql/test/library-tests/regex/test.cpp with the Ruby regex
corpus mechanically translated to C++ string literals and wrapped in
std::regex construction calls using fully self-contained in-file stubs
(no #include of any external/standard header).

Stub surface:
  std::basic_regex<CharT>  constructor(const char*), constructor(const char*, int),
                           assign(const char*)
  std::regex               typedef for basic_regex<char>
  std::regex_match / regex_search / regex_replace  free functions

1:1 corpus mapping from regexp.rb:
  - All Ruby /regex/ literals translated to C++ "string" literals
    (each backslash doubled: /\d/ -> "\\d")
  - Dropped: /#{A}bc/ (Ruby string interpolation, no string-literal form)
  - Kept: a{,8}, .*m, \A, \z, \G, \h\H, (?'foo'...) — these are removed
    in commits 4 and 5 (keeping them here preserves 1:1 mapping)

Also removes `abstract` from RegExp class in ParseRegExp.qll so that all
StringLiterals are regex candidates (trivial syntactic gate; no dataflow).

.expected generated by:
  codeql test run --learn --search-path=. cpp/ql/test/library-tests/regex/
  (CodeQL CLI 2.26.1, bundled C/C++ extractor)
All 2 tests passed.

Author: Copilot

cpp/ql/lib/semmle/code/cpp/regex/internal/ParseRegExp.qll

@@ -11,8 +11,12 @@ private import semmle.code.cpp.exprs.Literal
  * A C++ string literal that contains a regular expression term, that is, either
  * a regular expression literal, or a string literal used in a context where
  * it is parsed as a regular expression.
+ *
+ * All string literals are considered potential regexes for now. Future work
+ * will restrict this to only string literals actually used as regular
+ * expressions (via construction-site flow analysis).
  */
-abstract class RegExp extends StringLiteral {
+class RegExp extends StringLiteral {
   /**
    * Holds if this `RegExp` has the `s` flag for multi-line matching.
    */