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/classes/PStringUtils.cpp

@@ -65,23 +65,34 @@ std::vector<std::string> PStringUtils::Split(const std::string &str, const std::
 // IsInt (static)
 //--------------------------------------------------------------------------
 /**
- * <p>Returns true if the string is a non-empty sequence of decimal digits
- * only. Mirrors the semantics of TString::IsDigit().
+ * <p>Returns true if the string is an integer literal, i.e. a non-empty
+ * sequence of decimal digits with an optional single leading sign (+/-).
+ * This is slightly more permissive than TString::IsDigit(), which rejects a
+ * sign, so that negative/positive integers such as "-5" or "+42" are 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
  */
 bool PStringUtils::IsInt(const std::string &str)
 {
-  // mirror TString::IsDigit(): all characters must be digits or whitespace,
-  // and there must be at least one digit (surrounding/embedded whitespace is
-  // tolerated, e.g. for tokens split on ',' or ';' only).
+  // all characters must be digits or whitespace, with an optional single
+  // leading sign (+/-) preceding the digits, and there must be at least one
+  // digit (surrounding/embedded whitespace is tolerated, e.g. for tokens
+  // split on ',' or ';' only).
   bool hasDigit = false;
+  bool hasSign  = false;
   for (char c : str) {
-    if (std::isdigit(static_cast<unsigned char>(c)))
+    if (std::isdigit(static_cast<unsigned char>(c))) {
       hasDigit = true;
-    else if (!std::isspace(static_cast<unsigned char>(c)))
+    } else if (c == '+' || c == '-') {
+      // a sign is only valid before any digit and may appear at most once
+      if (hasDigit || hasSign)
+        return false;
+      hasSign = true;
+    } else if (!std::isspace(static_cast<unsigned char>(c))) {
       return false;
+    }
   }
   return hasDigit;
 }