Auth: pass the OIDC issuer through untouched, trailing slash and all

Deploying against the real Authentik turned this up immediately: its issuer
ends in a slash, OIDC requires the discovered issuer to match the configured
one byte-for-byte, and trimming it made discovery fail every time. The stub
in the tests happened to advertise a slashless issuer, so the whole suite
passed while the only provider Petal actually talks to could not be reached.
The stub now takes its issuer as a knob, and a regression test runs the flow
against one that ends in a slash.

Claude-Session: https://claude.ai/code/session_016y6gyuHkQXPiEuW8RGQyua

internal/auth/oidc_test.go

@@ -29,6 +29,10 @@ type stubIdP struct {
 	*httptest.Server
 	key      *rsa.PrivateKey
 	clientID string
+	// issuer as advertised by discovery and asserted in tokens. Defaults to the
+	// server's URL; a test can give it a trailing slash, which is what Authentik
+	// does and which OIDC requires to match byte-for-byte.
+	issuer string
 
 	// Claims the next token exchange will assert.
 	sub, email, name string
@@ -48,11 +52,12 @@ func newStubIdP(t *testing.T, clientID string) *stubIdP {
 
 	mux := http.NewServeMux()
 	idp.Server = httptest.NewServer(mux)
+	idp.issuer = idp.URL
 	t.Cleanup(idp.Close)
 
 	mux.HandleFunc("/.well-known/openid-configuration", func(w http.ResponseWriter, _ *http.Request) {
 		_ = json.NewEncoder(w).Encode(map[string]any{
-			"issuer":                                idp.URL,
+			"issuer":                                idp.issuer,
 			"authorization_endpoint":                idp.URL + "/authorize",
 			"token_endpoint":                        idp.URL + "/token",
 			"jwks_uri":                              idp.URL + "/jwks",
@@ -91,7 +96,7 @@ func (idp *stubIdP) idToken(t *testing.T) string {
 		t.Fatal(err)
 	}
 	payload, _ := json.Marshal(map[string]any{
-		"iss":   idp.URL,
+		"iss":   idp.issuer,
 		"aud":   idp.clientID,
 		"sub":   idp.sub,
 		"email": idp.email,
@@ -329,6 +334,40 @@ func TestCallbackHandlesProviderError(t *testing.T) {
 	assertNoSession(t, sessions, rec)
 }
 
+// Authentik's issuer ends in a slash, and OIDC requires the discovered issuer to
+// match the configured one byte-for-byte. Normalising it away made discovery
+// fail against the real provider while every stub test still passed.
+func TestDiscoveryKeepsTrailingSlashIssuer(t *testing.T) {
+	sessions, users, _ := newStores(t)
+	idp := newStubIdP(t, "petal")
+	idp.issuer = idp.URL + "/"
+	idp.sub, idp.email = "sub-her", "her@example.com"
+
+	o := NewOIDC(context.Background(), Options{
+		IssuerURL:    idp.issuer,
+		ClientID:     "petal",
+		ClientSecret: "shh",
+		BaseURL:      "http://petal.test",
+	}, sessions, users)
+	flow := o.Routes()
+
+	// A failed discovery renders the 503 "sign-in is unavailable" page instead
+	// of redirecting, so reaching the provider at all is the assertion.
+	target, jar := start(t, flow)
+	if !strings.HasPrefix(target.String(), idp.URL+"/authorize") {
+		t.Fatalf("login went to %q, want the provider's authorize endpoint", target)
+	}
+
+	// And the ID token it issues, whose `iss` carries the same slash, verifies.
+	idp.nonce = jar[nonceCookie]
+	rec := httptest.NewRecorder()
+	flow.ServeHTTP(rec, jar.attach(
+		httptest.NewRequest(http.MethodGet, "/callback?code=abc&state="+url.QueryEscape(jar[stateCookie]), nil)))
+	if rec.Code != http.StatusFound {
+		t.Fatalf("callback status=%d body=%s", rec.Code, rec.Body)
+	}
+}
+
 // Signing in when already signed in shouldn't bounce a good session through the
 // identity provider.
 func TestLoginSkipsWhenAlreadySignedIn(t *testing.T) {