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/server/contacts.py

@@ -1,4 +1,5 @@
 import logging
+from datetime import date
 from typing import Any
 
 from mcp.server.fastmcp import Context, FastMCP
@@ -83,7 +84,9 @@ def _raw_contact_to_model(raw: dict) -> Contact:
         uid=raw["vcard_id"],
         fn=contact_info.get("fullname", ""),
         etag=raw.get("getetag"),
-        birthday=contact_info.get("birthday"),
+        birthday=contact_info["birthday"].isoformat()
+        if isinstance(contact_info.get("birthday"), date)
+        else contact_info.get("birthday"),
         emails=emails,
         phones=phones,
         custom_fields=custom_fields,