feat(a2a): upgrade @ag-ui/a2a and @ag-ui/a2a-middleware to A2A Protocol v1.0 (@a2a-js/sdk@1.x)

## Summary

Upgrade the official AG-UI ↔ A2A bridge packages to **A2A Protocol Specification v1.0**, by moving from `@a2a-js/sdk@^0.2.x` to `@a2a-js/sdk@1.x`.

Today:

| Package | Declared dependency | Wire format in converters/tests |
| --- | --- | --- |
| `@ag-ui/a2a` (`0.0.6`) | `@a2a-js/sdk@^0.2.2` | v0.3-style (`kind: "text"`, `status-update`, etc.) |
| `@ag-ui/a2a-middleware` | `@a2a-js/sdk@^0.2.2` | same |

Meanwhile:

- A2A Protocol **v1.0.0** shipped (2026-03)
- Official `@a2a-js/sdk` is at **1.0.x** and implements the v1.0 data model / transports
- New A2A agents (e.g. Python SDK samples) speak v1.0 by default (unified `Part` without `kind`, `ROLE_*` / `TASK_STATE_*` enums, wrapped stream events, `supportedInterfaces`, etc.)

As a result, `@ag-ui/a2a` cannot cleanly talk to a pure v1.0 A2A peer without a compatibility layer or dual-version agent.

> **Status note:** `@ag-ui/a2a` README currently marks the integration as **Experimental**. This issue proposes graduating the wire-format support to A2A v1.0 while keeping the package stability expectations clear.

## Motivation

- A2A v1.0 is the stable production protocol line
- AG-UI docs position A2A as the Agent↔Agent layer complementary to AG-UI; the bridge should track the stable A2A line
- Avoid forcing every AG-UI host to pin remote agents to v0.3 forever

## Proposed scope

### In scope
1. Bump `@a2a-js/sdk` to `^1.0.0` (or latest 1.x) in:
- `integrations/a2a/typescript`
- `middlewares/a2a-middleware`
2. Update converters in `integrations/a2a/typescript/src/utils.ts` (and related types/tests) for v1.0 breaking changes, at least:
- `Part`: member-based content (`text` / `raw` / `url` / `data`) instead of `kind`
- Roles / task states: `ROLE_*`, `TASK_STATE_*`
- Stream event discrimination: wrapped members (`task` / `message` / `statusUpdate` / `artifactUpdate`) instead of top-level `kind`
- Agent Card: prefer `supportedInterfaces[]` (keep reading legacy fields only if needed for compat)
3. Update unit tests + dojo A2A examples as needed
4. Document supported A2A protocol version(s) in the package README

### Compatibility question (please advise)
Should the bridge:
- **A)** Speak **v1.0 only**, or
- **B)** Support **v1.0 + v0.3** (e.g. choose interface from Agent Card `protocolVersion`, or rely on `@a2a-js/sdk` compat helpers)?

My preference is **B** if maintainable, so existing v0.3 agents keep working during migration.

### Out of scope (unless maintainers want it bundled)
- Broader A2AAgent feature gaps already tracked elsewhere (shared state / HITL / richer Task-Artifact mapping) — see #762, #1938, #1968
- Designing a new push-notification transport — see #2106

## Related issues
- #762 — missing AG-UI features on A2AAgent bridge
- #1938 — `RunAgentInput.state` dropped
- #1968 — forward context/tools through `@ag-ui/a2a`
- #2106 — pushNotifications / A2A PushNotificationConfig alignment

None of the above explicitly tracks the **protocol v1.0 / SDK 1.x upgrade**.

## Willingness to contribute

I can take this if maintainers agree on scope + compatibility strategy.

Please assign me (or point to anyone already working on it) and tag code owners as appropriate (`@ag-ui-protocol/copilotkit`).

## Checklist before implementation
- [ ] Confirm no duplicate work already in progress
- [ ] Agree on compat strategy (v1.0-only vs dual-version)
- [ ] Agree on package versioning / changelog expectations for `@ag-ui/a2a` and `@ag-ui/a2a-middleware`

Thanks!