fix: reactive Activate/Deactivate buttons in plugin-list (closes #179) - #204

Summary

After clicking the Activate (or Deactivate) button on a plugin row in an `ability_result` message, the button label and its target action would stay frozen at the original state. So an Activate click left an "Activate" button on a now-active plugin — clicking it again hit "plugin is already active" instead of deactivating it.

Net: +71 / -20 lines, 2 files touched.

What changed

`MessageItem.jsx`

• Extracted the inline button JSX into a small `` component
• Tracks its own `loading` (shows `…`, button disabled) and `override` state
• After a successful click, looks up the action in a new `INVERSE_ACTIONS` map:
  ```js
  const INVERSE_ACTIONS = {
  'wp-agentic-admin/plugin-activate': { action: 'plugin-deactivate', button_label: 'Deactivate' },
  'wp-agentic-admin/plugin-deactivate': { action: 'plugin-activate', button_label: 'Activate' },
  };
  ```
• If the action has an inverse, the button flips to it (same plugin slug in `args`, just opposite ability id + label). Adding another reversible pair only needs a new entry in the map.

`ChatContainer.jsx`

• `handleAction` now `return`s the action result (was `return undefined`). ActionButton reads `result.success` and only flips on real successes — a failed activation doesn't lie about the state.
• Cancelled confirmations now return `{ success: false, cancelled: true }` instead of undefined.

Behavior matrix

Verification

• `npm test` — 96 passing, unchanged
• `npm run build` — clean
• `composer lint` — clean
• `npx wp-scripts lint-js` — clean (2 pre-existing ChatContainer warnings are not introduced here)

Manual test plan

• In the running Playground, send "list plugins"
• Click Activate on Akismet → label flips to Deactivate, click it → flips back to Activate
• Click Activate on a non-existent plugin (force-edit the action args) → label does NOT flip (error path)
• During click, see `…` placeholder on the button

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