Adds record_start()/record_stop()/record_status() to new
src/xceadmin/src/xceadmin/scheduler.py, following push_config()'s exact
pipeline shape (load_schema -> follow_redirects -> decode_reply). Deliberately
diverges from push_config()'s "return failure as data" convention because
this task's own Observable text requires raising a typed error on an error
reply, not just connection/decode failure -- a new SchedAdminError
(xcecommon/errors.py) carries the full reply state (error/recording/file/
event_count/started_at) when SchedAdminReply.ok is false.
test_scheduler.py mirrors test_config_push_mock.py's real mock-TLS-server
technique, covering all three actions, redirect-following (standby-to-
primary), and the error-reply-raises-SchedAdminError path across the
error codes reference table.
188 passed, 3 skipped via the real xceadmin test runner (meson compile
-C xce.build xceadmin).
