test(slack-config): cover credentials, start/stop, reactions, notify, kill-all

Sibling `test-slack-config-reactions.el` covered the post-command-hook safety advice and the empty-buffer guard. This batch covers the rest:

- `cj/slack--get-credential`: string secret, function secret, missing entry.
- `cj/slack-start`: errors when token / cookie missing, registers + starts when both present.
- `cj/slack-stop`: calls `slack-ws-close` + messages.
- `cj/slack--reaction-candidates`: includes the (Other...) escape hatch.
- `cj/slack-select-reaction`: emoji-name return for a curated pick, delegation to `slack-message-reaction-input` for Other.
- `cj/slack-notify`: fires for IMs (not self), skips self-messages and non-IM non-mentions.
- `cj/slack-test-notify`: fires the notify pipeline directly.
- `cj/slack-mark-read-and-bury`: marks via the latest-ts + buries; outside slack, just buries.
- `cj/slack-close-all-buffers`: closes buffers with `slack-current-buffer` local, leaves non-slack buffers alone.

The close-all assertions check identity (the 2 slack buffers killed, the non-slack one preserved) instead of an exact total -- ERT-internal buffers leaking into `(buffer-list)` from earlier tests would otherwise inflate the count, and `cl-letf` over the `buffer-list` subr is unreliable under native-comp.

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