[web_server_idf] Skip multipart parser sources when OTA upload is disabled

esphome/components/web_server_idf/__init__.py

@@ -1,5 +1,6 @@
 from esphome.components.esp32 import add_idf_sdkconfig_option
 import esphome.config_validation as cv
+from esphome.core import CORE
 
 CODEOWNERS = ["@dentra"]
 
@@ -12,3 +13,11 @@ CONFIG_SCHEMA = cv.All(
 async def to_code(config):
     # Increase the maximum supported size of headers section in HTTP request packet to be processed by the server
     add_idf_sdkconfig_option("CONFIG_HTTPD_MAX_REQ_HDR_LEN", 1024)
+
+
+def FILTER_SOURCE_FILES() -> list[str]:
+    # multipart.cpp is fully #ifdef'd on USE_WEBSERVER_OTA (set by the
+    # web_server OTA platform); skip it when OTA uploads are not configured.
+    if not any(define.name == "USE_WEBSERVER_OTA" for define in CORE.defines):
+        return ["multipart.cpp"]
+    return []