fix: add retry logic to OAuth2 metadata tests to avoid race conditions (#19813)

This PR adds a readiness wait to OAuth2 metadata endpoint tests to avoid rare races with server startup. Instead of immediately making HTTP requests, the tests now use `testutil.Eventually` to retry the requests until they succeed, with a short interval between attempts. This helps prevent flaky tests that might fail due to timing issues during server initialization.  
  
Fixes: https://github.com/coder/internal/issues/996

testutil/http.go

@@ -0,0 +1,39 @@
+package testutil
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// RequireEventuallyResponseOK makes HTTP GET requests to the given endpoint until it returns
+// 200 OK with a valid JSON response that can be decoded into target, or until the context
+// times out. This is useful for waiting for HTTP servers to become ready during tests,
+// especially for metadata endpoints that may not be immediately available.
+func RequireEventuallyResponseOK(ctx context.Context, t testing.TB, endpoint string, target interface{}) {
+	t.Helper()
+
+	ok := Eventually(ctx, t, func(ctx context.Context) (done bool) {
+		req, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
+		if err != nil {
+			return false
+		}
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			return false
+		}
+		defer resp.Body.Close()
+		if resp.StatusCode != http.StatusOK {
+			return false
+		}
+		if err := json.NewDecoder(resp.Body).Decode(target); err != nil {
+			return false
+		}
+		return true
+	}, IntervalFast)
+
+	require.True(t, ok, "endpoint %s not ready in time", endpoint)
+}