Make image paste handling more robust in the chat composer

## Summary
The chat composer image-paste path should be made more robust before release.

## Follow-ups
- Avoid relying on stale SwiftUI focus state where possible.
- Revisit the synchronous AppKit-to-SwiftUI notification bridge used for paste handling.
- Consider a dedicated AppKit-backed composer control or responder hook that can handle image paste directly.
- Ensure plain text paste always passes through normally.
- Add manual or automated coverage for focused composer paste, unfocused composer paste, image clipboard paste, and text clipboard paste.

## Context
The current implementation intercepts `Cmd+V` at the WordPress Agent window level and asks the SwiftUI composer to handle image paste only when focused. It is practical for testing, but a dedicated responder path would be cleaner and less fragile.