[GPCAPIM-395]: Preliminary implementation of in-memory APIM APP Auth API stub

Author: DWolfsNHS

gateway-api/stubs/stubs/apim_app_auth/__init__.py

@@ -0,0 +1,58 @@
+"""
+In-memory APIM APP Auth API stub.
+
+The stub does **not** implement the full APIM APP Auth API surface.
+"""
+
+from typing import Any
+
+from requests import Response
+
+from stubs.base_stub import PostStub, StubBase
+
+
+class APIMAppAuthStub(StubBase, PostStub):
+    def __init__(self) -> None:
+        self._post_url: str = ""
+        self._post_headers: dict[str, str] = {}
+        self._post_data: str = ""
+        self._post_timeout: int | None = None
+
+    @property
+    def post_url(self) -> str:
+        return self._post_url
+
+    @property
+    def post_headers(self) -> dict[str, str]:
+        return self._post_headers
+
+    @property
+    def post_data(self) -> str:
+        return self._post_data
+
+    @property
+    def post_timeout(self) -> int | None:
+        return self._post_timeout
+
+    # TODO: validation?
+
+    def post(
+        self,
+        url: str,
+        data: str,
+        **kwargs: Any,  # noqa: ARG002 - kwargs are required to match subclass signature
+    ) -> Response:
+        self._post_url = url
+        self._post_data = data
+
+        response = self._create_response(
+            status_code=200,
+            json_data={
+                "access_token": "access_token",
+                "expires_in": "599",
+                "token_type": "Bearer",
+                "issued_at": "1777366854223",
+            },
+        )
+
+        return response