[web_server_idf] Respect explicit HTTP header limit (#18657)

Co-authored-by: jeroen85 <26403565+jeroen85@users.noreply.github.com>
Co-authored-by: Jeroen Jansen <jeroen@Jeroens-MacBook-Air.local>

tests/component_tests/web_server/test_httpd_sdkconfig.py

@@ -0,0 +1,32 @@
+"""Tests for ESP-IDF web server sdkconfig defaults."""
+
+from collections.abc import Callable
+
+from esphome.components.esp32 import RawSdkconfigValue
+from esphome.components.esp32.const import KEY_ESP32, KEY_SDKCONFIG_OPTIONS
+from esphome.core import CORE
+
+
+def test_web_server_idf_uses_default_header_limit(
+    generate_main: Callable[[str], str],
+) -> None:
+    """The web server retains its existing 1024-byte default."""
+    generate_main("tests/component_tests/web_server/web_server_no_auth.yaml")
+
+    assert (
+        CORE.data[KEY_ESP32][KEY_SDKCONFIG_OPTIONS]["CONFIG_HTTPD_MAX_REQ_HDR_LEN"]
+        == 1024
+    )
+
+
+def test_web_server_idf_respects_explicit_header_limit(
+    generate_main: Callable[[str], str],
+) -> None:
+    """An explicit sdkconfig option takes precedence over the component default."""
+    generate_main(
+        "tests/component_tests/web_server/web_server_header_limit_override.yaml"
+    )
+
+    value = CORE.data[KEY_ESP32][KEY_SDKCONFIG_OPTIONS]["CONFIG_HTTPD_MAX_REQ_HDR_LEN"]
+    assert isinstance(value, RawSdkconfigValue)
+    assert value.value == "4096"

tests/component_tests/web_server/web_server_header_limit_override.yaml

@@ -0,0 +1,16 @@
+---
+esphome:
+  name: test
+
+esp32:
+  board: nodemcu-32s
+  framework:
+    type: esp-idf
+    sdkconfig_options:
+      CONFIG_HTTPD_MAX_REQ_HDR_LEN: "4096"
+
+wifi:
+  ssid: MySSID
+  password: password1
+
+web_server: