test: dedizierter mcptest-User statt Stefans echter Daten

USER_ALIASES test->mcptest. Eigene Backends pro Dienst:
- Radicale-User mcptest + /mcptest/calendar-test + contacts-test
- oCIS-User mcptest (Graph-API), Joplin lokales Profil :41186
- statische Test-Maildir tests/testdata/maildir (mcp-test-mail/-empty)
test_all.py auf mcptest-Backends umgestellt (Account-/Kalender-/Kontaktnamen).
config.json (gitignored) mit mcptest in allen Maps. Doku: tests/MCPTEST.md.
54 Tests gruen. Stefans calendar-test/contacts-test + Joplin-Test-Reste entfernt.

tests/testdata/maildir/mcp-test-mail/INBOX/cur/1781847419.M881146P1986110Q2.polo:2,S

@@ -0,0 +1,23 @@
+Content-Type: multipart/mixed; boundary="===============0575628907158327116=="
+MIME-Version: 1.0
+From: billing@example.com
+To: mcptest@local
+Subject: Ihre Rechnung Juni 2026
+Date: Tue, 17 Jun 2026 10:00:00 +0200
+
+--===============0575628907158327116==
+Content-Type: text/plain; charset="utf-8"
+MIME-Version: 1.0
+Content-Transfer-Encoding: base64
+
+SW0gQW5oYW5nIGZpbmRlbiBTaWUgSWhyZSBSZWNobnVuZy4=
+
+--===============0575628907158327116==
+Content-Type: application/pdf
+MIME-Version: 1.0
+Content-Transfer-Encoding: base64
+Content-Disposition: attachment; filename="rechnung.pdf"
+
+JVBERi0xLjQKMSAwIG9iajw8Pj5lbmRvYmoKdHJhaWxlcjw8Pj4KJSVFT0YK
+
+--===============0575628907158327116==--