fix(esp_http_server): report truncation when header value length matches buffer size

httpd_req_get_hdr_value_str() detected truncation with `val_size < full_size`, where full_size is the strlcpy() return value. strlcpy() returns strlen() of the source (the terminating null is not counted), so truncation actually occurs when strlen(src) >= val_size. At strlen(src) == val_size the value is copied as val_size - 1 chars + NUL (i.e. truncated) yet ESP_OK was returned, so the caller never learned the value was cut.

Use `val_size <= full_size` and correct the misleading comment about strlcpy()'s return value.

Same truncation-reporting class fixed for httpd_cookie_key_value in PR #16202; httpd_req_get_hdr_value_str was missed. No memory-safety impact: strlcpy() null-terminates if val_size > 0.

components/esp_http_server/src/httpd_parse.c

@@ -1150,12 +1150,13 @@ esp_err_t httpd_req_get_hdr_value_str(httpd_req_t *r, const char *field, char *v
     }
 
     /* Get the NULL terminated value and copy it to the caller's buffer.
-     * Note `strlcpy()` will always return the size of the source string
-     * including terminating null. */
+     * strlcpy() returns strlen() of the source string (the terminating
+     * null is NOT counted), so only val_size - 1 characters fit. */
     size_t full_size = strlcpy(val, val_ptr, val_size);
 
-    /* If buffer length is smaller than needed, return truncation error */
-    if (val_size < full_size) {
+    /* If the value did not fit in the buffer it was truncated, i.e. its
+     * length reached or exceeded val_size. Return truncation error. */
+    if (val_size <= full_size) {
         return ESP_ERR_HTTPD_RESULT_TRUNC;
     }
     return ESP_OK;