Update esphome/core/helpers.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

esphome/core/helpers.cpp

@@ -373,10 +373,8 @@ int8_t step_to_accuracy_decimals(float step) {
   return str.length() - dot_pos - 1;
 }
 
-// Use C-style string constant to store in ROM instead of RAM (saves 24 bytes)
-static constexpr const char *BASE64_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-                                            "abcdefghijklmnopqrstuvwxyz"
-                                            "0123456789+/";
+// Store BASE64 characters as array - automatically placed in flash/ROM on embedded platforms
+static const char BASE64_CHARS[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
 // Helper function to find the index of a base64 character in the lookup table.
 // Returns the character's position (0-63) if found, or 0 if not found.
@@ -386,8 +384,8 @@ static constexpr const char *BASE64_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 // stops processing at the first invalid character due to the is_base64() check in its
 // while loop condition, making this edge case harmless in practice.
 static inline uint8_t base64_find_char(char c) {
-  const char *pos = strchr(BASE64_CHARS, c);
-  return pos ? (pos - BASE64_CHARS) : 0;
+  const void *ptr = memchr(BASE64_CHARS, c, sizeof(BASE64_CHARS));
+  return ptr ? (static_cast<const char *>(ptr) - BASE64_CHARS) : 0;
 }
 
 static inline bool is_base64(char c) { return (isalnum(c) || (c == '+') || (c == '/')); }