V03Compat: silent regression for v0.3 clients sending `configuration` without explicit `blocking: true`

## Summary

After upgrading a server from a2a-dotnet v0.3 to v1 with the V03Compat layer, v0.3 clients that send a `configuration` field (without explicitly setting `blocking: true`) silently lose the final agent message and any artifacts enqueued after task completion. The response returns at the first Task event (state `Submitted`) with no final data, and no exception is raised.

## Root cause

1. `V03TypeConverter.ToV1SendMessageRequest` maps `ReturnImmediately = !cfg.Blocking` (`src/A2A.V0_3Compat/V03TypeConverter.cs:500`).
2. The v0.3 `MessageSendConfiguration.Blocking` property defaults to `false` and is serialized unconditionally (non-nullable `bool`, default `JsonIgnoreCondition.WhenWritingNull` — `src/A2A.V0_3/A2AJsonUtilities.cs:55`).
3. The v0.3 server SDK's `TaskManager.SendMessageAsync` never read `Blocking` — it always awaited `OnTaskCreated` (`src/A2A.V0_3/Server/TaskManager.cs:144-210`). Historically, every v0.3 client got blocking semantics regardless of what they sent.

Net result: any v0.3 client that sends `configuration` without `blocking: true` now gets `ReturnImmediately = true` through the compat layer. The v1 server breaks on the first Task event (`src/A2A/Server/A2AServer.cs:638-641`), which in typical handler patterns is the `Submitted` snapshot from `TaskUpdater.SubmitAsync`. Subsequent `Working`, artifact, and `Completed` events are applied to the task store in the background but do not reach the `SendMessage` response on this call.

## Requests

1. **Other known v0.3 SDK consumers** — we've already notified the maintainers of `work-iq-cli` (the one v0.3 client we're aware of), and they will patch it to set `blocking: true`. Do you know of other teams or projects building on `A2A.V0_3` that we should reach? Since the failure mode is silent (no exception, missing data), we'd like to proactively notify any we can before they hit this in production.

2. **Release notes / breaking-change entry on V03Compat** — for consumers we can't reach directly, a note calling out the `Blocking` semantic change (from "ignored" in v0.3 SDK to "honored" in v1 via compat) would let them discover the required change on their own upgrade path.

## What we're doing on our side

Notified the maintainers of `work-iq-cli`; they will patch it to send `blocking: true`. That's the only v0.3 consumer on our radar, which is why we'd like help identifying others.

The issue is in the V03Compat layer, specifically in `src/A2A.V0_3Compat/V03TypeConverter.cs` around line 500. The root cause involves understanding the serialization behavior in `src/A2A.V0_3/A2AJsonUtilities.cs:55` and the v0.3 server SDK's `TaskManager.SendMessageAsync` in `src/A2A.V0_3/Server/TaskManager.cs`. A fix must ensure backward compatibility for v0.3 clients. Testing requires knowledge of the A2A protocol and the interaction between v0.3 and v1 semantics.

Rédigé par le modèle d'indexation à partir du texte de l'issue.