MCP header button: opt-in browser session exposure for third-party agents

[cstns]

Third-party MCP agents can only operate on browser tabs that the user has explicitly exposed. Add an MCP button to the page header (similar to the existing Expert button in ExpertButton.vue / PageHeader.vue) that toggles third-party session exposure for the current tab.

When toggled ON:

• Register the tab's session in the Redis cache (Store browser session presence in Forge's Redis cache #8158) with the first heartbeat
• Start the heartbeat interval and context-update watchers
• Open an MQTT connection using the tab session ID (Add a tab session ID to the context store for third-party MCP targeting #8157) and subscribe to in-flight topics so third-party agents can reach this tab
• Show a visual indicator that the session is exposed

When toggled OFF (or tab closes):

• Clear the session from the cache
• Stop heartbeats and context watchers
• Unsubscribe from in-flight topics
• The MQTT connection for first-party chat (if any) is unaffected

The button should be gated by the same AI platform feature flags that gate the Expert button. The toggle state can be persisted to sessionStorage so a page refresh within the same tab restores the exposure without requiring the user to click again.

Depends on #8157 and #8158.