fix(collection): open collections created while heartbeat is stale

[lazerg]

Fix #16524

Summary

The Collection constructor only calls onOpen() if conn.readyState is connected. That getter reports disconnected when the last heartbeat is stale, even though _readyState never changed. A model compiled in that window never gets its collection opened, and since no state transition happens later, the automatic ensureIndexes() sits in the collection queue forever and Model.init() never settles.

The fix checks _readyState in the constructor instead, the same check useDb() uses to decide whether it can wire up right away. Buffered connection-level ops (like the _waitForConnect() in init()) still wait for the heartbeat to recover, as #16203 set up.

Examples

Added a test in test/connection.test.js that compiles a model with a unique index while the heartbeat is stale, emits a heartbeat, then checks that init() resolves and name_1 exists. It times out on master. The repro script from the issue also prints settled and [ '_id_', 'name_1' ] with this change.

[vkarpov15]

@lazerg can you please take a look at #16527?