fix(site/e2e): accept 404 from external auth reset hook (#26793)

> 🤖 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`
2. #26793 `fix(site/e2e): accept 404 from external auth reset hook` ←
this PR
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`

`deleteExternalAuthByID` used to be inverted: `sql.ErrNoRows` (link
doesn't exist for this user/provider) fell through to the `500` path,
while non-`ErrNoRows` DB errors went to `httpapi.ResourceNotFound`.
#19775 (Sep 2025) refactored it to return `404` for not-found and `500`
for real DB errors, which is the contract you'd expect.

The relevant lines from #19775 in `coderd/externalauth.go`:

```diff
-	err := api.Database.DeleteExternalAuthLink(ctx, ...)
+	link, err := api.Database.GetExternalAuthLink(ctx, ...)
 	if err != nil {
-		if !errors.Is(err, sql.ErrNoRows) {
+		if errors.Is(err, sql.ErrNoRows) {
 			httpapi.ResourceNotFound(w)
 			return
 		}
 		httpapi.Write(ctx, w, http.StatusInternalServerError, ...)
 		return
 	}
```

`resetExternalAuthKey` in `site/e2e/hooks.ts` still treats `500` as the
not-found code, so the first `beforeEach` in the externalAuth suite
throws. The suite was skipped at the time #19775 landed (#17235), so
nobody noticed the contract drift until #26648 tried to re-enable it.

This just flips the accepted status codes to `200 || 404` and rewrites
the stale comment. The 401/403/500 paths still surface as failures,
which is what we want.

Refs https://linear.app/codercom/issue/DEVEX-413
Refs https://github.com/coder/coder/pull/19775

<details>
<summary>Why a separate PR</summary>

Keeps the bisection signal clean: #26575 proves the EADDRINUSE flake is
fixed, this PR fixes the hook contract drift surfaced by re-enabling the
suite, and #26648 just flips `.skip`. Squashing into #26648 would
conflate two unrelated fixes.

The CI run on #26648 already confirms the flake fix is doing its job:
`successful external auth from workspace` passes (5.6s) and the
`beforeAll`/`afterAll` mock servers come up and tear down cleanly with
no EADDRINUSE. The only failures are this 404 hook drift.

</details>

site/e2e/hooks.ts

@@ -71,8 +71,8 @@ export const resetExternalAuthKey = async (context: BrowserContext) => {
 		});
 
 		res.on("end", () => {
-			// Both 200 (key deleted successfully) and 500 (key was not found) are valid responses.
-			if (res.statusCode !== 200 && res.statusCode !== 500) {
+			// 200 = link deleted; 404 = no link existed for this provider.
+			if (res.statusCode !== 200 && res.statusCode !== 404) {
 				console.error("failed to delete external auth link", data);
 				throw new Error(
 					`failed to delete external auth link: HTTP response ${res.statusCode}`,