Add MakeSecureString helper, eliminate .c_str() in password paths

- Add MakeSecureString(const std::string&) in allocators.h
- Replace .c_str() shims in walletpassphrase, walletpassphrasechange,
  encryptwallet RPCs and askpassphrasedialog
- Update TODO_DOCUMENTATION.md to mark issue as resolved

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/allocators.h

@@ -254,4 +254,9 @@ struct zero_after_free_allocator : public std::allocator<T>
 // This is exactly like std::string, but with a custom allocator.
 typedef std::basic_string<char, std::char_traits<char>, secure_allocator<char> > SecureString;
 
+static inline SecureString MakeSecureString(const std::string& value)
+{
+    return SecureString(value.begin(), value.end());
+}
+
 #endif