feat: add support for 'features.agent_view' app manifest property

[mwbrooks]

Changelog

Add support for the new app manifest property features.agent_view. This prevents the CLI from silently dropping agent_view when reading and writing a manifest.

Summary

Supports the upcoming features.agent_view in the Slack app manifest. Without these struct changes the CLI silently drops agent_view when reading and writing a manifest.

• Mirrors the existing AssistantView shape and reuses the existing SuggestedPrompts type because agent_view and assistant_view are sibling properties.
• The CLI delegates manifest validation to the backend, so no validator changes are needed - these struct edits ensure the field round-trips cleanly through JSON and YAML.

Related: slackapi/manifest-schema#74

Preview

• N/A

Testing

# Create a new app
$ ./bin/slack create my-app -t slack-samples/bolt-js-starter-agent --subdir claude-agent-sdk
$ cd my-app/

$ vim manifest.json
# Add the following under "assistant_view":
$ cat manifest.json
    "assistant_view": {
      "assistant_description": "Hi, I am an agent built using Bolt for JavaScript. I am here to help you out!",
      "suggested_prompts": []
    },
    "agent_view": {
      "agent_description": "Hi, I am an agent built using Bolt for JavaScript. I am here to help you out!",
      "suggested_prompts": [],
      "actions": [{
        "name": "open_settings",
        "description": "Open the agent settings panel."
      }]
    },

# Test that the `agent_view` survives reading:
$ ../bin/slack manifest info --source local
# → Confirm "agent_view": { ...

# Clean up
$ cd ../
$ rm -rf my-app/

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.67%. Comparing base (0fcc9e6) to head (6014806).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #596   +/-   ##
=======================================
  Coverage   71.67%   71.67%           
=======================================
  Files         226      226           
  Lines       19176    19176           
=======================================
  Hits        13745    13745           
  Misses       4220     4220           
  Partials     1211     1211           

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