test(session): tamper inside the signature, not its last character

The last base64url character of a 32-byte HMAC signature partly carries
padding bits that decoders ignore, so the tampered token was sometimes
still valid and the test failed at random. Changing a character in the
middle of the signature always alters the signature bytes.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

src/lib/session.test.ts

@@ -70,8 +70,14 @@ describe("Session-Token", () => {
       role: "KUNDE",
       sessionVersion: 0,
     });
-    // Letztes Zeichen der Signatur veraendern.
-    const tampered = token.slice(0, -1) + (token.endsWith("A") ? "B" : "A");
+    // Ein Zeichen MITTEN in der Signatur veraendern. Das letzte Zeichen taugt
+    // nicht: bei 32 Signaturbytes traegt es in Base64url teils nur Fuellbits,
+    // die der Decoder ignoriert — der Test schlug dann zufaellig fehl, obwohl
+    // die Signatur korrekt geprueft wurde.
+    const signatureStart = token.lastIndexOf(".") + 1;
+    const position = signatureStart + 10;
+    const replacement = token[position] === "A" ? "B" : "A";
+    const tampered = token.slice(0, position) + replacement + token.slice(position + 1);
 
     await expect(verifySessionToken(tampered)).resolves.toBeNull();
   });