feat(esp_eth): update test_apps to use Ethernet test component

Move test logic into the eth_test_app managed component and slim down
the test_apps main target and pytest suite accordingly.

components/esp_eth/test_apps/conftest.py

@@ -0,0 +1,46 @@
+# SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: Apache-2.0
+import sys
+from collections.abc import Callable
+from pathlib import Path
+from typing import TYPE_CHECKING
+
+import pytest
+
+if TYPE_CHECKING:
+    from eth_test_runner import EthTestRunner
+
+
+def _setup_eth_test_runner_sys_path() -> Path | None:
+    root = Path(__file__).resolve().parent
+    for name in ('espressif__eth_test_app', 'eth_test_app'):
+        candidate = root / 'managed_components' / name
+        if (candidate / 'eth_test_runner.py').exists():
+            candidate_str = str(candidate)
+            if candidate_str not in sys.path:
+                sys.path.insert(0, candidate_str)
+            return candidate
+
+    return None
+
+
+# Support local runs when managed_components is already present after build.
+_setup_eth_test_runner_sys_path()
+
+
+@pytest.fixture
+def eth_test_runner(
+    app_path: str,
+    download_app_extra: Callable[[str], None],
+) -> 'EthTestRunner':
+    runner_dir = _setup_eth_test_runner_sys_path()
+    if runner_dir is None:
+        download_app_extra(app_path)
+        runner_dir = _setup_eth_test_runner_sys_path()
+
+    if runner_dir is None:
+        raise RuntimeError('eth_test_runner.py not found. Build the test app locally or download CI artifacts first.')
+
+    from eth_test_runner import EthTestRunner
+
+    return EthTestRunner()