Replaced mock data in the Slack connector's channel management UI with a real API call to the backend.
- Implemented `discoverSlackChannelsAPI` in the `useConnectorEditPage` hook to fetch channels from `/api/search-source-connectors/slack/:connectorId/discover-channels`.
- Updated the `handleDiscoverChannels` function in `EditConnectorPage` to use this new API function.
- Ensured consistent `SlackChannelInfo` type usage.
This fixes a bug where incorrect Slack channel names were displayed due to the UI using stale mock data instead of actual data reflecting the bot's true channel memberships.