fix(site/e2e): close mock external-auth servers in teardown (#26575)

> 🤖 This PR was written by Coder Agents on behalf of Jake Howell.

Stack:
1. #26575 `fix(site/e2e): close mock external-auth servers in teardown`
← this PR
2. #26793 `fix(site/e2e): accept 404 from external auth reset hook`
3. #26795 `fix(site/src): refresh provider state after device-flow
exchange`
4. #26798 `fix(site/e2e): reset both providers in external auth hook`
5. #26648 `chore(site/e2e): re-enable externalAuth suite`

The externalAuth e2e suite has been skipped since #17235 because
`createServer` in `site/e2e/helpers.ts` started an express server but
never gave callers a way to close it. On retries or repeated runs, the
listener from the previous invocation was still bound to the hardcoded
port and the next `beforeAll` failed with `EADDRINUSE`, eventually
timing out in `waitForPort`.

`createServer` now returns a `{ app, close }` pair. The web flow closes
in `afterAll`; the device flow uses `try/finally`.
`closeAllConnections()` is called before `close()` so teardown stays
bounded if keep-alive connections linger.

The suite remains `test.describe.skip` here; #26648 flips the skip off
once the rest of the stack is in.

Refs https://linear.app/codercom/issue/DEVEX-413
Refs https://github.com/coder/internal/issues/356

<details>
<summary>Decision log</summary>

Discussed the full options list with @jakehwll before drafting. Picked
option A (minimal teardown) because:

- Two prior PRs (#15537, #16528) attacked symptoms (port probing, longer
timeout) without addressing the leaked listener.
- Kayla's diagnosis on coder/internal#356 pointed at exactly this case:
nothing else in CI is grabbing the port, the listener from the previous
run is still bound.
- A is mechanical and orthogonal: it adds a real teardown without
changing port allocation, fixture wiring, or what gets mocked. If the
flake persists after A, we know to escalate to a worker-scoped fixture
or dynamically allocated ports.

Returning `close` rather than the raw `http.Server` encapsulates the
`closeAllConnections` + `close` choreography so callers don't repeat it.
`closeAllConnections` is optional-chained because it landed in Node
18.2; coder/coder runs newer, but the chain costs nothing.

The device test uses `try/finally` rather than a shared `afterEach` to
keep per-test state local. The web flow's `afterAll` mirrors its
`beforeAll`.

</details>

site/e2e/tests/externalAuth.spec.ts

@@ -14,8 +14,11 @@ import { beforeCoderTest, resetExternalAuthKey } from "../hooks";
 
 test.describe
 	.skip("externalAuth", () => {
+		let closeWebServer: (() => Promise<void>) | undefined;
+
 		test.beforeAll(async ({ baseURL }) => {
-			const srv = await createServer(gitAuth.webPort);
+			const { app: srv, close } = await createServer(gitAuth.webPort);
+			closeWebServer = close;
 
 			// The GitHub validate endpoint returns the currently authenticated user!
 			srv.use(gitAuth.validatePath, (_req, res) => {
@@ -34,6 +37,10 @@ test.describe
 			});
 		});
 
+		test.afterAll(async () => {
+			await closeWebServer?.();
+		});
+
 		test.beforeEach(async ({ context, page }) => {
 			beforeCoderTest(page);
 			await login(page);
@@ -51,43 +58,49 @@ test.describe
 			};
 
 			// Start a server to mock the GitHub API.
-			const srv = await createServer(gitAuth.devicePort);
-			srv.use(gitAuth.validatePath, (_req, res) => {
-				res.write(JSON.stringify(ghUser));
-				res.end();
-			});
-			srv.use(gitAuth.codePath, (_req, res) => {
-				res.write(JSON.stringify(device));
-				res.end();
-			});
-			srv.use(gitAuth.installationsPath, (_req, res) => {
-				res.write(JSON.stringify(ghInstall));
-				res.end();
-			});
+			const { app: srv, close: closeServer } = await createServer(
+				gitAuth.devicePort,
+			);
+			try {
+				srv.use(gitAuth.validatePath, (_req, res) => {
+					res.write(JSON.stringify(ghUser));
+					res.end();
+				});
+				srv.use(gitAuth.codePath, (_req, res) => {
+					res.write(JSON.stringify(device));
+					res.end();
+				});
+				srv.use(gitAuth.installationsPath, (_req, res) => {
+					res.write(JSON.stringify(ghInstall));
+					res.end();
+				});
 
-			const token = {
-				access_token: "",
-				error: "authorization_pending",
-				error_description: "",
-			};
-			// First we send a result from the API that the token hasn't been
-			// authorized yet to ensure the UI reacts properly.
-			const sentPending = new Awaiter();
-			srv.use(gitAuth.tokenPath, (_req, res) => {
-				res.write(JSON.stringify(token));
-				res.end();
-				sentPending.done();
-			});
+				const token = {
+					access_token: "",
+					error: "authorization_pending",
+					error_description: "",
+				};
+				// First we send a result from the API that the token hasn't been
+				// authorized yet to ensure the UI reacts properly.
+				const sentPending = new Awaiter();
+				srv.use(gitAuth.tokenPath, (_req, res) => {
+					res.write(JSON.stringify(token));
+					res.end();
+					sentPending.done();
+				});
 
-			await page.goto(`/external-auth/${gitAuth.deviceProvider}`, {
-				waitUntil: "domcontentloaded",
-			});
-			await page.getByText(device.user_code).isVisible();
-			await sentPending.wait();
-			// Update the token to be valid and ensure the UI updates!
-			token.error = "";
-			token.access_token = "hello-world";
-			await page.waitForSelector("text=1 organization authorized");
+				await page.goto(`/external-auth/${gitAuth.deviceProvider}`, {
+					waitUntil: "domcontentloaded",
+				});
+				await page.getByText(device.user_code).isVisible();
+				await sentPending.wait();
+				// Update the token to be valid and ensure the UI updates!
+				token.error = "";
+				token.access_token = "hello-world";
+				await page.waitForSelector("text=1 organization authorized");
+			} finally {
+				await closeServer();
+			}
 		});
 
 		test("external auth web", async ({ page }) => {