fix(site/src/pages/AgentsPage): persist empty MCP selection (#28238)

Removing the final optional MCP server from an existing chat produced an
empty selection, but the message request omitted `mcp_server_ids`. The
API interprets an omitted field as preserving the current selection.

Send the selected MCP server IDs for every message, including an empty
array. Add a Storybook interaction test that removes the final MCP
server and verifies the request contains `mcp_server_ids: []`.

<details>
<summary>Manual verification on a local dev instance</summary>

Setup: `./scripts/develop.sh`, an Anthropic provider with
`claude-haiku-4-5`, and a local test MCP server registered with
availability `default_on`.

With this branch, chat `75285d9f`:

1. The new chat showed the MCP chip selected.
2. Sent a message, then removed the chip with the X control.
3. Sent a second message, then reloaded the page.
4. No MCP chip appeared, and the picker toggle stayed off.
5. `GET /api/experimental/chats/{id}` returned `mcp_server_ids: []`.

With the one-line change reverted, chat `751ac191` repeated the same
flow. The chip returned as selected after the reload, and the API
returned `mcp_server_ids: ["b63a2a3a-..."]`.

Not covered: `force_on` servers, plan mode interaction, and queued
messages during streaming.

</details>

Generated by Coder Agents.

site/src/pages/AgentsPage/AgentChatPage.stories.tsx

@@ -26,6 +26,7 @@ import {
 	MockChat,
 	MockChatMessage,
 	MockChatQueuedMessage,
+	MockMCPServerConfig,
 } from "#/testHelpers/chatEntities";
 import { MockChatModelConfig } from "#/testHelpers/chatModels";
 import {
@@ -255,7 +256,10 @@ const buildChatAuthorizationQuery = (
 const buildQueries = (
 	chat: TypesGen.Chat,
 	messagesData: TypesGen.ChatMessagesResponse,
-	opts?: { diffUrl?: string },
+	opts?: {
+		diffUrl?: string;
+		mcpServers?: readonly TypesGen.MCPServerConfig[];
+	},
 ) => {
 	const diffStatus: TypesGen.ChatDiffStatus = {
 		chat_id: CHAT_ID,
@@ -301,7 +305,7 @@ const buildQueries = (
 		},
 		{ key: chatModelsKey, data: mockModelCatalog },
 		{ key: chatModelConfigs().queryKey, data: mockModelConfigs },
-		{ key: mcpServersKey, data: [] },
+		{ key: mcpServersKey, data: opts?.mcpServers ?? [] },
 		buildChatAuthorizationQuery(chat, {
 			canShareChat: {
 				action: "share",
@@ -3217,6 +3221,53 @@ export const SendResponseAfterChatSwitch: Story = {
 	},
 };
 
+export const RemoveLastMCPServer: Story = {
+	parameters: {
+		queries: buildQueries(
+			{
+				id: CHAT_ID,
+				...baseChatFields,
+				title: "Remove last MCP server",
+				status: "waiting",
+				mcp_server_ids: [MockMCPServerConfig.id],
+			},
+			{ messages: [], queued_messages: [], has_more: false },
+			{
+				diffUrl: undefined,
+				mcpServers: [MockMCPServerConfig],
+			},
+		),
+	},
+	beforeEach: () => {
+		spyOn(API.experimental, "getUserSkills").mockResolvedValue([]);
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		const sendSpy = spyOn(
+			API.experimental,
+			"createChatMessage",
+		).mockResolvedValue({ queued: false });
+
+		await userEvent.click(
+			await canvas.findByRole("button", { name: "Remove MCP Server" }),
+		);
+		const editor = await canvas.findByTestId("chat-message-input");
+		await userEvent.click(editor);
+		await userEvent.type(editor, "Send without MCP tools");
+		await userEvent.keyboard("{Enter}");
+
+		await waitFor(() => {
+			expect(sendSpy).toHaveBeenCalledTimes(1);
+		});
+		expect(sendSpy).toHaveBeenCalledWith(
+			CHAT_ID,
+			expect.objectContaining({
+				mcp_server_ids: [],
+			}),
+		);
+	},
+};
+
 /**
  * The send flow renders the durable user row once the server accepts the
  * prompt, before the assistant turn produces any output.

site/src/pages/AgentsPage/AgentChatPage.tsx

@@ -1799,10 +1799,7 @@ const AgentChatPage: FC = () => {
 			content,
 			model_config_id: selectedModelConfigID,
 			reasoning_effort: effectiveReasoningEffort,
-			mcp_server_ids:
-				effectiveMCPServerIds.length > 0
-					? [...effectiveMCPServerIds]
-					: undefined,
+			mcp_server_ids: [...effectiveMCPServerIds],
 			...(planModeSwitch !== undefined
 				? {
 						plan_mode: