Fix Slack invalid_blocks error when usage metadata is empty

[rhegisan]

Summary

This PR fixes a Slack invalid_blocks error caused by rendering a Block Kit
context element with an empty text field.

Details

When usage_info is empty or unavailable, the Slack API rejects the message with:

Error Details: The request to the Slack API failed. (url: https://slack.com/api/chat.postMessage, status: 200)
The server responded with: {'ok': False, 'error': 'invalid_blocks', 'errors': ['must provide a string [json-pointer:/blocks/4/elements/0/text]']}

This change conditionally renders the usage metadata block only when valid
text is present, preventing message failures and improving bot stability.

Scope

• ✅ Fixes Slack UI rendering errors
• ❌ Does not change agent behavior, A2A responses, or Kubernetes logic

Testing

• Verified /mykagent command no longer fails with invalid_blocks
• Messages render correctly with and without usage metadata