Explore and Improve handling page life cycle state changes in FDC3 for Web / getAgent()

## Enhancement Request

### Use Case

Modern browsers manage page lifecycles (which can throttle or halt JavaScript execution within an app window when it is hidden). Events are available through the [Page Lifecycle browser API](https://developer.chrome.com/docs/web-platform/page-lifecycle-api) which inform the application of changes in its status.

Further to #1703, the behaviour of the DesktopAgentProxy via the WCP and DACP protocols in FDC3 for Web Browsers should be examined and any issues that occur when apps enter throttled (hidden) or frozen states identified and solutions proposed.

The [Browser Desktop Agent specs](https://fdc3.finos.org/docs/api/specs/browserResidentDesktopAgents) do not currently address app state changes nor recommend behaviour for Desktop Agents during state changes - which they should to promote consistent behaviour across different Desktop Agent implementations.

See #1686 for the original discussion of the issue

Start with the Browser Desktop Agent specs and the Page Lifecycle browser API, then review the related discussions in #1703 and #1686. Examine how DesktopAgentProxy behavior over WCP and DACP should account for hidden, throttled, or frozen apps, and define the documented recommendations and solutions needed for consistent Desktop Agent implementations.

Written by the indexing model from the issue text.