[Bug]: A2UI action retries generate different synthetic message identities

## Describe the Bug

A2UIMiddleware assigns fresh synthetic assistant, Tool call and result IDs each time it processes an action. Retrying the same run therefore produces a different backend request, preventing exact replay and potentially repeating the action.

## Steps to Reproduce

1. Create a RunAgentInput with a runId and forwardedProps.a2uiAction.
2. Process that input through A2UIMiddleware and record the synthetic message pair sent downstream.
3. Retry the same input with the same runId.
4. Compare the synthetic assistant message ID, Tool call ID and Tool result ID: all change.

## Expected Behavior

A retry of the same run preserves its synthetic identities. A new action uses a new runId and receives different identities even when the payload is identical.

## Environment

TypeScript A2UI middleware on current main; Node 22.23.1. The proposed fix is limited to action message creation and focused middleware tests.

Inspect the A2UIMiddleware implementation where synthetic assistant, tool call, and tool result IDs are created during RunAgentInput processing. Review existing middleware tests that cover action message creation and retry behavior, focusing on forwardedProps.a2uiAction with a repeated runId. Add or extend a focused test to replay the same input twice and assert stable IDs when runId is unchanged, and changed identities for a new runId. Verify the relevant middleware test suite passes.

Scritto dal modello di indicizzazione a partire dal testo della issue.