[web_server_idf] Remove std::string temporaries from multipart header parsing (#13940)

esphome/components/web_server_idf/utils.cpp

@@ -98,8 +98,8 @@ bool str_ncmp_ci(const char *s1, const char *s2, size_t n) {
   return true;
 }
 
-// Case-insensitive string search (like strstr but case-insensitive)
-const char *stristr(const char *haystack, const char *needle) {
+// Bounded case-insensitive string search (like strcasestr but length-bounded)
+const char *strcasestr_n(const char *haystack, size_t haystack_len, const char *needle) {
   if (!haystack) {
     return nullptr;
   }
@@ -109,7 +109,12 @@ const char *stristr(const char *haystack, const char *needle) {
     return haystack;
   }
 
-  for (const char *p = haystack; *p; p++) {
+  if (haystack_len < needle_len) {
+    return nullptr;
+  }
+
+  const char *end = haystack + haystack_len - needle_len + 1;
+  for (const char *p = haystack; p < end; p++) {
     if (str_ncmp_ci(p, needle, needle_len)) {
       return p;
     }