test: use default provider in TestPatchChatMessage/ChangesModel (#25189)

`TestPatchChatMessage/ChangesModel` hardcoded `"openai"` as the provider
for the override model config. After #25171, the shared chat test
harness registers a single `"openai-compat"` provider by default, so
calling `createAdditionalChatModelConfig(..., "openai", ...)` fails with
HTTP 400 `Chat provider is not configured` before the test can exercise
the model-change path. The subtest was added in #25084 after #25171 was
reviewed, so the harness change and the new hardcoded provider only met
on `main`.

Use `defaultModel.Provider` so the override always matches whatever
provider the harness registered. This mirrors every other call site of
`createAdditionalChatModelConfig` in the file.

Closes https://github.com/coder/internal/issues/1530

coderd/exp_chats_test.go

@@ -7290,7 +7290,7 @@ func TestPatchChatMessage(t *testing.T) {
 		overrideModel := createAdditionalChatModelConfig(
 			t,
 			client,
-			"openai",
+			defaultModel.Provider,
 			"gpt-4o-mini-edit-override",
 		)
 

site/src/pages/AgentsPage/components/ChatConversation/messageHelpers.test.ts

@@ -19,6 +19,7 @@ const getDisplayState = (message: ChatMessage) =>
 		message,
 		parsed: parseMessageContent(message.content),
 		hideActions: false,
+		hasActiveStream: false,
 	});
 
 describe("deriveMessageDisplayState", () => {