Keep caller supplied default headers in async_create_clientsession (#179824)

Co-authored-by: Claude <noreply@anthropic.com>

homeassistant/helpers/aiohttp_client.py

@@ -16,6 +16,7 @@ from aiohttp import ClientMiddlewareType, hdrs, web
 from aiohttp.hdrs import CONTENT_TYPE, USER_AGENT
 from aiohttp.web_exceptions import HTTPBadGateway, HTTPGatewayTimeout
 from aiohttp_asyncmdnsresolver.api import AsyncDualMDNSResolver
+from multidict import CIMultiDict
 from yarl import URL
 
 from homeassistant import config_entries
@@ -300,8 +301,10 @@ def _async_create_clientsession(
     # It's important that we identify as Home Assistant
     # If a package requires a different user agent, override it by passing a headers
     # dictionary to the request method.
+    default_headers = CIMultiDict(clientsession.headers)
+    default_headers[USER_AGENT] = SERVER_SOFTWARE
     clientsession._default_headers = MappingProxyType(  # type: ignore[assignment]  # noqa: SLF001
-        {USER_AGENT: SERVER_SOFTWARE},
+        default_headers
     )
 
     clientsession.close = warn_use(  # type: ignore[method-assign]

tests/helpers/test_aiohttp_client.py

@@ -393,6 +393,17 @@ async def test_client_session_immutable_headers(hass: HomeAssistant) -> None:
         session.headers.update({"user-agent": "bla"})
 
 
+async def test_create_clientsession_keeps_headers(hass: HomeAssistant) -> None:
+    """Test default headers passed to the session are kept."""
+    session = client.async_create_clientsession(
+        hass, headers={"Authorization": "Basic bla", "User-Agent": "bla"}
+    )
+
+    assert session.headers["authorization"] == "Basic bla"
+    # We always identify as Home Assistant
+    assert session.headers["user-agent"] == client.SERVER_SOFTWARE
+
+
 @pytest.mark.usefixtures("disable_mock_zeroconf_resolver")
 @pytest.mark.usefixtures("mock_async_zeroconf")
 async def test_async_mdnsresolver(