Add Slack blocks support to post_message

[suhaibmujahid]

Resolves #6669

Extend Slack message actions to accept optional blocks payloads.

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[Copilot]

Pull request overview

Extends Hackbot’s Slack action plumbing to support Slack Block Kit payloads on recorded slack.post_message actions, enabling richer message layouts while keeping text as a fallback.

Changes:

• Add blocks recording support on the Slack action recording side (tests included).
• Pass recorded blocks through to chat.postMessage in the apply-side Slack handler.
• Add apply-side test coverage for posting recorded blocks while keeping text as a fallback.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
libs/hackbot-runtime/tests/test_slack_handler.py	Adds an apply-side test asserting blocks are posted and text remains as fallback.
libs/hackbot-runtime/tests/test_slack_actions.py	Adds recording-side tests for blocks behavior and validation around text/blocks.
libs/hackbot-runtime/hackbot_runtime/actions/slack.py	Updates parameter shaping to optionally include blocks and adds blocks support to record_message.
libs/hackbot-runtime/hackbot_runtime/actions/handlers/slack_handler.py	Threads optional blocks into the Slack SDK chat_postMessage call.

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