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/session/export_test.go

@@ -1,10 +0,0 @@
-package session
-
-import "github.com/gorilla/sessions"
-
-// NewStore exposes the production cookie-store constructor so tests
-// exercise the store the application actually runs with, rather than a
-// lookalike assembled in the test.
-func NewStore(key []byte) *sessions.CookieStore {
-	return newStore(key)
-}

internal/session/testing.go

@@ -8,6 +8,13 @@ import (
 	"sneak.berlin/go/webhooker/internal/config"
 )
 
+// NewStore exposes the production cookie-store constructor so tests
+// exercise the store the application actually runs with, rather than a
+// lookalike assembled in the test.
+func NewStore(key []byte) *sessions.CookieStore {
+	return newStore(key)
+}
+
 // NewForTest creates a Session with a pre-configured cookie store for use
 // in tests. This bypasses the fx lifecycle and database dependency, allowing
 // middleware and handler tests to use real session functionality. The key