fix: preserve A2A transcript order

[EItanya]

Summary

• archive input-required status messages before user replies
• publish tool calls and results as A2A status messages instead of artifacts
• remove the UI transcript-order inference and replay protocol history directly

Testing

• go test ./core/v2/a2agateway ./adk/pkg/a2a
• yarn test src/api/chat/a2aGrpcChatClient.test.ts
• yarn typecheck
• yarn eslint src/api/chat/a2aGrpcChatClient.ts src/api/chat/a2aGrpcChatClient.test.ts

Closes #2584

[EItanya]

Design context: why conversion happens in KAgentExecutor

We traced the exact ADK Go v2.2.0 path and audited its callback/plugin surfaces before settling on the executor decorator.

The upstream path is:

session.Event -> part conversion -> private eventToArtifactTransform -> TaskArtifactUpdateEvent

Except for long-running calls that become input-required, every content-bearing ADK event is passed through the artifact transform. Both supported output modes produce artifacts.

The available hooks do not provide a persistence-safe event-type replacement:

Hook	Limitation
Model callbacks	Modify model requests/responses, not A2A event types
Tool callbacks	Modify tool arguments/results only
Agent callbacks	Add or replace agent content, which is still converted to an artifact
Plugin OnEventCallback	Can replace a session.Event, but artifact conversion happens afterwards
GenAIPartConverter	Converts individual parts only
A2A AfterEventCallback	Receives an already-created TaskArtifactUpdateEvent and cannot replace it
Custom runner/provider	Still feeds session events into the same artifact-only processor
A2A call interceptor	Can replace the outbound streamed event, but runs after the task store processed the original artifact, so replay would remain wrong

Marking a tool as long-running is only correct when execution must enter input-required; using it for ordinary tool calls/results would change task semantics.

Therefore the AgentExecutor decorator is the earliest public seam where kagent can replace the concrete A2A event before the A2A SDK persists it. The cleaner long-term solution is upstream support for an event converter that returns a general a2a.Event (or an artifact/message/status transform), at which point this local conversion can be removed.