Components.AI: Correct shell semantics, dismissal, and focus behavior

### Is your feature request related to a problem? Please describe the problem.

Drawer and bubble are labeled as dialogs despite being non-modal, lack complete Escape behavior, and do not consistently restore or place focus.

### Describe the solution you'd like

Use labeled region/complementary semantics, expose `aria-expanded`/`aria-controls`, focus the composer on open, restore the trigger on close, support Escape cancellation/dismissal, and never trap focus. Decide whether `popover="manual"` should provide bubble top-layer behavior.

### Additional context

Leaf task for #68227 requirements H3-H5 and open design decision 5.

Start with the Drawer and bubble components and the requirements in #68227, especially H3-H5 and open design decision 5. Verify the intended semantics, expanded/controlled relationships, focus movement and restoration, Escape behavior, focus non-trapping, and whether popover="manual" should provide bubble top-layer behavior; done means these behaviors are implemented consistently.

Written by the indexing model from the issue text.