The `get_all_channels` method in `slack_history.py` was making paginated
requests to `conversations.list` without any delay, leading to HTTP 429
errors when fetching channels from large Slack workspaces.
This commit introduces the following changes:
- Adds a 3-second delay between paginated calls to `conversations.list`
to comply with Slack's Tier 2 rate limits (approx. 20 requests/minute).
- Implements handling for the `Retry-After` header when a 429 error is
received. The system will wait for the specified duration before
retrying. If the header is missing or invalid, a default of 60 seconds
is used.
- Adds comprehensive unit tests to verify the new delay and retry logic,
covering scenarios with and without the `Retry-After` header, as well
as other API errors.
