PStringUtils::IsInt: accept an optional leading sign

IsInt now recognises (possibly signed) integers such as "-5" or "+42",
making it slightly more permissive than TString::IsDigit(). A lone sign,
a double sign, or a sign following a digit are still rejected. strToNum
test expectations updated accordingly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/include/PStringUtils.h

@@ -67,11 +67,13 @@ class PStringUtils
     static std::vector<std::string> Split(const std::string &str, const std::string &delimiters);
 
     /**
-     * <p>Returns true if the string is a non-empty sequence of decimal
-     * digits only. Mirrors TString::IsDigit().
+     * <p>Returns true if the string is a (possibly signed) integer literal,
+     * i.e. a non-empty sequence of decimal digits with an optional single
+     * leading sign (+/-). Slightly more permissive than TString::IsDigit(),
+     * which rejects a sign, so that e.g. "-5" is also recognised.
      *
      * @param str string to be checked
-     * @return true if str consists of digits only
+     * @return true if str is a (possibly signed) integer
      */
     static bool IsInt(const std::string &str);