Build the middleware test cookie stores with the production constructor (closes #154)

The middleware tests built their cookie stores by setting store.Options by hand, which left the securecookie codecs at the library's 30-day default instead of the 7-day cap production sets, an invisible divergence that would outlive the next change to store construction. The test store constructor moves from internal/session/export_test.go into internal/session/testing.go so other packages can reach it, and the two middleware test helpers build their stores through it. No test in the repo builds a cookie store by hand any more, and no assertion changes.

Model: opus-5-5

internal/middleware/middleware_test.go

@@ -12,7 +12,6 @@ import (
 	"testing"
 	"time"
 
-	"github.com/gorilla/sessions"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
@@ -78,14 +77,7 @@ func newTestSessionManager(
 		key[i] = byte(i)
 	}
 
-	store := sessions.NewCookieStore(key)
-	store.Options = &sessions.Options{
-		Path:     "/",
-		MaxAge:   86400 * 7,
-		HttpOnly: true,
-		Secure:   false,
-		SameSite: http.SameSiteLaxMode,
-	}
+	store := session.NewStore(key)
 
 	var now func() time.Time
 
@@ -931,8 +923,7 @@ func metricsAuthMiddleware(
 	}
 
 	key := make([]byte, testKeySize)
-	store := sessions.NewCookieStore(key)
-	store.Options = &sessions.Options{Path: "/", MaxAge: 86400}
+	store := session.NewStore(key)
 
 	sessManager := session.NewForTest(store, cfg, log, key, nil)