Slack's send() caught all exceptions and returned a bare
SendResult(success=False) — never setting retryable=True or extracting
the server's Retry-After header. When Slack returned a 429 rate-limit
error, the base _send_with_retry() layer saw retryable=False and did
not retry, silently dropping remaining message chunks.
Reuse the existing _is_retryable_upload_error() helper (which already
detects 429, 500+, and connection-type errors) to set retryable=True,
and extract the Retry-After header from the SlackApiError response
when present so the base retry layer honors Slack's backoff schedule
instead of its own default.
Sibling of the Telegram FloodWait fix (PR #46762 / commit 404b06ac4)
which added the SendResult.retry_after plumbing to the base layer.
Adds five regression tests covering 429 with/without Retry-After,
500 server errors, 403 non-retryable errors, and connection errors.
5243fcafa1