[core] Add portable strcasestr implementation named str_contains_ignore_case (#18497)

Co-authored-by: J. Nick Koston <nick@koston.org>

esphome/core/helpers.cpp

@@ -220,6 +220,19 @@ bool str_endswith_ignore_case(const char *str, size_t str_len, const char *suffi
   return strncasecmp(str + str_len - suffix_len, suffix, suffix_len) == 0;
 }
 
+bool str_contains_ignore_case_fallback(const char *haystack, const char *needle) {
+  const size_t needle_len = strlen(needle);
+  if (needle_len == 0) {
+    return true;
+  }
+  for (const char *p = haystack; *p != '\0'; p++) {
+    if (strncasecmp(p, needle, needle_len) == 0) {
+      return true;
+    }
+  }
+  return false;
+}
+
 // str_truncate, str_until, str_lower_case, str_upper_case, str_snake_case moved to alloc_helpers.cpp
 char *str_sanitize_to(char *buffer, size_t buffer_size, const char *str) {
   if (buffer_size == 0) {

esphome/core/helpers.h

@@ -981,6 +981,25 @@ inline bool str_endswith_ignore_case(const std::string &str, const char *suffix)
   return str_endswith_ignore_case(str.c_str(), str.size(), suffix, strlen(suffix));
 }
 
+/// Fallback implementation for case insensitive substring comparison.
+bool str_contains_ignore_case_fallback(const char *haystack, const char *needle);
+
+/// Case-insensitive check if needle string is contained in haystack (no heap allocation).
+inline bool str_contains_ignore_case(const char *haystack, const char *needle) {
+  if (!needle || !haystack) {
+    return false;
+  }
+
+// strcasestr is a GNU extension: newlib only declares it when _GNU_SOURCE is set.
+// ESP32/ESP8266/host builds get it from their framework or from g++ on Linux;
+// LibreTiny, RP2 and Zephyr do not, so they use the hand-rolled fallback.
+#if defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
+  return str_contains_ignore_case_fallback(haystack, needle);
+#else   // defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
+  return strcasestr(haystack, needle) != nullptr;
+#endif  // defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
+}
+
 // str_truncate moved to alloc_helpers.h - remove this include before 2026.11.0
 
 // str_until, str_lower_case, str_upper_case moved to alloc_helpers.h - remove this comment before 2026.11.0