fix(remoteagent/v2): invoke AfterA2ARequestCallbacks on synthesized aggregated events

[nuthalapativarun]

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Link to Issue or Description of Change

• Closes: remoteagent/v2: AfterA2ARequestCallbacks are not invoked on the aggregated event synthesized from partial artifact chunks #948

Root Cause

aggregatePartial synthesizes new non-partial events via buildNonPartialAggregation when accumulated partial artifact chunks arrive before a terminal status update. These synthesized events were emitted directly via yield without going through runAfterA2ARequestCallbacks, so registered AfterA2ARequestCallback handlers never received them.

The original event (converted from the individual A2A chunk) correctly went through the callbacks, but the aggregated result—which is the event callers actually care about—did not.

Fix

In processEvent, after calling aggregatePartial, run runAfterA2ARequestCallbacks on each emitted event whose pointer differs from the original event (i.e., synthesized aggregated events). The original event already went through callbacks above the aggregatePartial call.

for _, toEmit := range processor.aggregatePartial(ctx, a2aEvent, event) {
    if toEmit != event {
        if cbResp, cbErr := processor.runAfterA2ARequestCallbacks(ctx, toEmit, nil); cbResp != nil || cbErr != nil {
            if cbErr != nil {
                return yieldErr(cbErr)
            }
            toEmit = cbResp
        }
    }
    if !yield(toEmit, nil) {
        return false
    }
}

Testing Plan

Unit Tests:

• Added TestRemoteAgent_AfterCallbackInvokedOnAggregatedEvent: streams two partial artifact chunks ("Hel" + "lo") followed by a terminal status. Registers a callback that counts invocations and records non-partial content text. Asserts 4 callback invocations (partial×2 + aggregated + terminal) and ["Hello"] as the text seen by the callback for the aggregated non-partial event.
• All existing tests in ./agent/remoteagent/v2/... continue to pass.

Manual End-to-End (E2E) Tests:
The fix is contained to the callback dispatch path in processEvent. The change is additive—existing behavior is preserved for the common case (single-event slice from aggregatePartial) and the new path only fires when a synthesized aggregated event is present.

Checklist

• Read CONTRIBUTING.md
• Self-reviewed code
• Added tests proving fix
• New and existing tests pass