Refactor: route formatter label fallback through internal helper (#1320)

This PR introduces a small internal refactor in `FormatterBase` by
routing label fallback through a dedicated helper.

There is no behavior change and no public API change:
- custom labels set via `label(...)` still take precedence
- default behavior remains unchanged

This is intended as a small first step toward making future
internationalization easier to integrate, as discussed in #1277.

---------

Co-authored-by: Omer Bulut <omer.bulut@asisguard.com.tr>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

include/CLI/FormatterFwd.hpp

@@ -71,6 +71,9 @@ class FormatterBase {
     /// Values are Needs, Excludes, etc.
     std::map<std::string, std::string> labels_{};
 
+    /// Default user-facing help labels used when no override is set
+    static std::string default_label(const std::string &key) { return key; }
+
     ///@}
     /// @name Basic
     ///@{
@@ -130,9 +133,8 @@ class FormatterBase {
 
     /// Get the current value of a name (REQUIRED, etc.)
     CLI11_NODISCARD std::string get_label(std::string key) const {
-        if(labels_.find(key) == labels_.end())
-            return key;
-        return labels_.at(key);
+        auto it = labels_.find(key);
+        return it != labels_.end() ? it->second : default_label(key);
     }
 
     /// Get the current left column width (options/flags/subcommands)