feat(2): mattermost-lts create-message round-trip (§4.3 P3) — first-user→login→team→channel→post→read-back; harness http.post_with_headers (returns response headers, for mattermost login Token)

runner/harness/http.py

@@ -132,6 +132,40 @@ def http_request(
         return 0, None
 
 
+def post_with_headers(
+    url: str,
+    data: dict | bytes | None = None,
+    headers: dict[str, str] | None = None,
+    content_type: str = "application/json",
+    timeout: int = 15,
+) -> tuple[int, object | None, dict[str, str]]:
+    """Like http_post but ALSO returns the response headers as a dict — for APIs that hand back an
+    auth token in a response header rather than the body (e.g. mattermost login → `Token` header).
+    Returns (status, parsed_json_or_None, response_headers). status=0 + {} on transport failure."""
+    if isinstance(data, (bytes, bytearray)):
+        body: bytes | None = bytes(data)
+    elif content_type == "application/json" and data is not None:
+        body = json.dumps(data).encode()
+    elif content_type == "application/x-www-form-urlencoded" and data is not None:
+        body = urllib.parse.urlencode(data).encode()
+    else:
+        body = None
+    req = urllib.request.Request(url, data=body, method="POST")
+    req.add_header("Content-Type", content_type)
+    for k, v in (headers or {}).items():
+        req.add_header(k, v)
+    try:
+        with urllib.request.urlopen(req, timeout=timeout, context=_CTX) as resp:
+            return resp.getcode(), _parse_body(resp.read()), dict(resp.headers)
+    except urllib.error.HTTPError as e:
+        try:
+            return e.code, _parse_body(e.read()), dict(e.headers or {})
+        except Exception:  # noqa: BLE001
+            return e.code, None, dict(getattr(e, "headers", {}) or {})
+    except Exception:  # noqa: BLE001
+        return 0, None, {}
+
+
 def assert_converges(
     fn,
     description: str,