Update tests for user mapping credentials and catalogs.conf resolution

- Move client_id/client_secret from server OPTIONS to CREATE USER MAPPING
  in all tests that previously used server-level credentials
- Add test_reject_secrets_in_server_options: verifies client_id in server
  OPTIONS is now rejected
- Add user mapping tests: valid options, PUBLIC mapping, rejection of
  unknown and server-level options in user mapping context
- Add catalogs.conf tests with pg_data_dir and catalogs_conf fixtures:
  credentials from config file, user mapping override, missing credentials
  error, and scope from config file
- Update ALTER SERVER test to use scope instead of client_id

Signed-off-by: sfc-gh-npuka <naisila.puka@snowflake.com>

Author: sfc-gh-npuka