Add metadata to Slack notifications

[suhaibmujahid]

Resolves #6671

Enhances Slack chat_postMessage calls to include structured message metadata for notification events. The metadata now carries run traceability details (run_id, agent name, and a direct Hackbot UI run URL), improving downstream context and linkage.

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[Copilot]

Pull request overview

Adds structured Slack message metadata to chat_postMessage calls so notification messages can be traced back to a specific Hackbot run (run id, agent, and a UI deep link), aligning with Issue #6671’s goal of making sent messages easier to interact with downstream.

Changes:

• Add a metadata payload to Slack chat_postMessage calls in the apply-side Slack handler.
• Update the Slack handler unit test to assert the new request payload (including metadata).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
libs/hackbot-runtime/hackbot_runtime/actions/handlers/slack_handler.py	Adds Slack metadata to outgoing chat_postMessage requests for run traceability.
libs/hackbot-runtime/tests/test_slack_handler.py	Updates assertions to expect the new Slack request shape including metadata.

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