a2ui generative-UI surfaces don't render over the AG-UI transport

[blove]

Summary

In examples/ag-ui (the standalone ag-ui chat example), a2ui / generative-UI surfaces do not render over the AG-UI transport. The chat message/streaming/markdown/error paths all work over ag-ui (8/9 e2e specs pass), but the a2ui surface does not.

Reproduction

examples/ag-ui/angular/e2e/a2ui-single-bubble.spec.ts (currently test.fixme):

• Prompt: "Demo: render a feedback form"
• Expected: an <a2ui-surface> rendered in a single assistant bubble.
• Actual: the render_a2ui_surface tool call is surfaced as a tool-call chip ("render_a2ui_surface Completed") — it never becomes an <a2ui-surface>.

The same graph + same aimock fixtures render the surface correctly over the LangGraph transport (examples/chat), so the graph emits the a2ui data correctly; the gap is in how the chat a2ui render path consumes it over ag-ui.

What we already did

• Added the customEvents signal to @threadplane/ag-ui (feat(ag-ui): customEvents signal for live a2ui streaming parity #606) so the chat composition's partial-args bridge can feature-detect it. This was necessary but not sufficient — the surface still doesn't render.

Hypotheses to investigate (one or more)

1. tool-views registry interception (feat(ag-ui): tool views — render frontend components for tool calls via the views registry #600/feat(cockpit): ag-ui/json-render — AG-UI shared-state generative UI #608): the new "ag-ui tool views" feature renders tool calls as components via a views registry. render_a2ui_surface may be caught as a generic tool-view chip, pre-empting the a2ui surface path.
2. wrapped-content emission: the chat a2ui "wrapped-content" classifier (libs/chat/.../content-classifier.ts, the ---a2ui_JSON--- path) expects the final surface in the assistant message content; over ag-ui that wrapper may not be produced.
3. partial-args bridge engagement: the live a2ui-partial custom-event path (now wired via customEvents) may not fire / reconstruct the surface over the uvicorn ag-ui-langgraph stream.

Scope

Likely a focused fix in libs/chat (a2ui render path) and/or libs/ag-ui, not an examples/ag-ui tweak. When fixed, flip test.fixme → test in a2ui-single-bubble.spec.ts.

Found during the examples/ag-ui Part 2b e2e port. Spec/design: docs/superpowers/specs/2026-06-06-examples-ag-ui-standalone-design.md.