fix: convert BDAY datetime.date to string before Pydantic validation

pythonvCard4 parses vCard BDAY fields into datetime.date objects, but
the Contact model expects Optional[str]. This caused a validation error
that crashed the entire contact list. Convert at the client layer
(consistent with the calendar client pattern) with a defensive check
at the server mapping layer.

Closes #672

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/contacts.py

@@ -2,6 +2,7 @@
 
 import logging
 import xml.etree.ElementTree as ET
+from datetime import date
 
 from pythonvCard4.vcard import Contact
 
@@ -272,7 +273,9 @@ class ContactsClient(BaseNextcloudClient):
                     "contact": {
                         "fullname": contact.fn,
                         "nickname": contact.nickname,
-                        "birthday": contact.bday,
+                        "birthday": contact.bday.isoformat()
+                        if isinstance(contact.bday, date)
+                        else contact.bday,
                         "email": contact.email,
                         "tel": contact.tel,
                     },