flake: e2e externalAuth.spec.ts

# Problem

See https://github.com/coder/internal/issues/212 for context.

```
==> Errors
/home/runner/work/coder/coder/site/e2e/tests/externalAuth.spec.ts:15:
13 | import { beforeCoderTest, resetExternalAuthKey } from "../hooks";
14 |
> 15 | test.beforeAll(async ({ baseURL }) => {
| ^
16 | const srv = await createServer(gitAuth.webPort);
17 |
18 | // The GitHub validate endpoint returns the currently authenticated user!
"beforeAll" hook timeout of 30000ms exceeded.
```

It looks like the hardcoded port (50516) can be unavailable for more than 30s.

# Proposed Solution

1) Simply bump the timeout (https://github.com/coder/coder/pull/16528)

2) Refactor the e2e tests to dynamically create a port.

Start in site/e2e/tests/externalAuth.spec.ts at the beforeAll hook on line 15, then review the createServer call, hardcoded port 50516, and the linked context. Reproduce the hook timeout and compare the proposed timeout change with dynamic port creation; done means the external-auth e2e setup no longer flakes when that port is unavailable.

Scritto dal modello di indicizzazione a partire dal testo della issue.