fix(auth): address PR #757 review feedback

- Branch the ProvisioningRequiredError message on the elicit result so a
  user who acknowledged the prompt isn't told to call
  nc_auth_provision_access (which would loop an LLM that just confirmed
  via elicitation). Other paths keep the existing instruction.
- Convert present_login_url's f-string logger.warning to lazy %s, matching
  present_provisioning_required and the repo's lazy-logging preference.
- Add a test for NEXTCLOUD_PUBLIC_ISSUER_URL trailing-slash normalization.
- Strengthen the decorator-elicits test: split into the "accepted" and
  "message_only" branches so the error-message change is regression-tested.

Refs: cbcoutinho/nextcloud-mcp-server#757#issuecomment-4363552487

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

nextcloud_mcp_server/auth/elicitation.py

@@ -115,8 +115,9 @@ async def present_login_url(
         return "message_only"
     except Exception as e:
         logger.warning(
-            f"Elicitation failed unexpectedly ({type(e).__name__}: {e}), "
-            "falling back to message"
+            "Elicitation failed unexpectedly (%s: %s), falling back to message",
+            type(e).__name__,
+            e,
         )
         return "message_only"