Adds platforms.slack.extra.native_task_cards: when enabled, live tool
calls render as Slack-native plan/task cards via chat.startStream /
chat.appendStream (task_display_mode: plan, task_update chunks) instead
of text/edit progress bubbles. ID-bearing tool_start/tool_complete
callbacks correlate concurrent same-name tool calls correctly; any
native API failure falls back to one continuously edited text update.
The stream is stopped exactly once when the turn finalizes.
Salvaged from PR #29496 onto current main (TurnRunner/TurnContext seam);
closes#29483.
eb137762ff