Two silent-loss modes in the ephemeral slash reply path:
1. Delivery failure was swallowed: _send_slash_ephemeral returned
success=True on any POST failure, so the user's actual command reply
vanished behind the stale 'Running /cmd…' ack. It now returns
success=False and send() falls back to normal channel delivery.
2. Long replies were truncated to the first ~39k chunk with no notice.
Replies are now chunked across response_url POSTs (first replaces
the ack, follow-ups append, all ephemeral), capped at Slack's 5-POST
response_url budget with an explicit truncation notice when exceeded.
Also updates the #55357 bounded-error-read test for the #26788 precise
context matching (ContextVar must be set) and channel fallback.
Fixes#19688
2a8d4879f2