ci: add API compatibility check job for merge requests

Add a check_api_compatibility CI job that uses esp-api-check to detect
breaking API changes in merge requests. The job compares API
declarations between the MR base and head commits using libclang, and
posts a discussion thread on the MR if WARNING or BREAKING changes are
found.

- Add CI job in host-test.yml with clang toolchain and allow_failure
- Add test_api_check project for CMake configuration with all
  components enabled
- Define __DOXYGEN__ and IDF_DOC_BUILD so the checker can see
  declarations behind these guards

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitlab/ci/host-test.yml

@@ -53,6 +53,25 @@ check_public_headers:
         done
       done
 
+check_api_compatibility:
+  extends:
+    - .host_test_template
+    - .rules:test:host_test
+  needs:
+    - pipeline_variables
+  variables:
+    IDF_TOOLCHAIN: clang
+    INSTALL_EXTRA_TOOLS: esp-clang-libs
+  allow_failure: true
+  script:
+    - pip install "esp-api-check[gitlab] @ git+https://gitlab-ci-token:${ESPCI_TOKEN}@${GITLAB_HTTPS_HOST}/espressif/esp-api-check.git"
+    - python -m esp_api_check install-clang-bindings
+    - python -m esp_api_check gitlab
+      --gitlab-url "https://${GITLAB_HTTPS_HOST}"
+      --gitlab-token ${ESPCI_TOKEN}
+      --head-sha ${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA}
+      --project-dir ${CI_PROJECT_DIR}/tools/test_apps/system/test_api_check
+
 test_nvs_coverage:
   extends:
     - .host_test_template