test(talk): cover include_status + malformed-header paths; drop Content-Type from default headers

Addresses the missing-test and Content-Type points from the latest
PR #741 review:

- client/talk.py _talk_headers(): drop the manual `Content-Type:
  application/json`. httpx sets it automatically on requests that pass
  `json=`, and we no longer leak it onto bodyless GETs and DELETEs.
- tests/client/talk/test_talk_api.py:
  - new `test_talk_list_participants_with_include_status` asserting
    `includeStatus=true` is forwarded.
  - new `test_talk_get_messages_invalid_last_given_header` covering
    the defensive try/except around the `X-Chat-Last-Given` parse —
    asserts the fallback `last_given=None` and that a warning is
    logged.
  - existing `test_talk_list_participants` extended to assert that
    `includeStatus` is *absent* by default.

Unit tests: 13 → 15. Integration tests still 7/7.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/talk.py

@@ -33,11 +33,16 @@ class TalkClient(BaseNextcloudClient):
     _CHAT_BASE = "/ocs/v2.php/apps/spreed/api/v1/chat"
 
     def _talk_headers(self) -> dict[str, str]:
-        """Standard OCS+JSON headers for spreed API calls."""
+        """Standard OCS+JSON headers for spreed API calls.
+
+        ``Content-Type`` is intentionally omitted — httpx adds it
+        automatically (and correctly) on requests that pass ``json=``,
+        so setting it here would also leak it onto bodyless GETs and
+        DELETEs.
+        """
         return {
             "OCS-APIRequest": "true",
             "Accept": "application/json",
-            "Content-Type": "application/json",
         }
 
     # Conversations (rooms)