test: use httptest server.Client() to isolate transport (#26409)

`TestPush/CachesSubscriptionsWithinTTL` could fail with `Post
"http://127.0.0.1:XXXXX": net/http: HTTP/1.x transport connection
broken: http: CloseIdleConnections called` when a sibling parallel
subtest's `httptest.Server.Close()` ran during an in-flight `Dispatch`.

`setupPushTestWithOptions` wired the dispatcher to `http.DefaultClient`,
so every parallel subtest in `TestPush` shared `http.DefaultTransport`.
`httptest.Server.Close()` calls `CloseIdleConnections` on
`http.DefaultTransport`, which could break an in-flight request in any
other subtest using the same transport.

`httptest.Server` already exposes a paired `*http.Client` backed by a
transport dedicated to that server (see `net/http/httptest/server.go`).
Closing one server only touches `http.DefaultTransport` and its own
client's transport, so sibling cleanup can no longer reach into ours.

Same flake class and same isolation principle as #25015, #25407, #25430,
and #25821.

Closes https://github.com/coder/internal/issues/1593
Closes ENG-2926

coderd/webpush/webpush_test.go

@@ -536,9 +536,7 @@ func setupPushTestWithOptions(ctx context.Context, t *testing.T, db database.Sto
 	server := httptest.NewServer(http.HandlerFunc(handlerFunc))
 	t.Cleanup(server.Close)
 
-	// Use an unrestricted HTTP client for tests. The default SSRF-safe
-	// client rejects loopback addresses, which blocks httptest.Server.
-	opts = append(opts, webpush.WithHTTPClient(http.DefaultClient))
+	opts = append(opts, webpush.WithHTTPClient(server.Client()))
 	manager, err := webpush.New(ctx, &logger, db, "http://example.com", opts...)
 	require.NoError(t, err, "Failed to create webpush manager")