ITK: v0.3/v1.0 SSE streaming wire format incompatibility

## Problem

The ITK test runner and v1.0 agents use incompatible SSE wire formats, which means streaming interop tests are not actually validating SSE event structure.

### Test runner (pydantic SDK, `a2a-sdk==0.3.25`)

Expects flat events with `kind` discriminator and lowercase state enums:

```json
data: {"kind": "status-update", "taskId": "tsk-1", "status": {"state": "working"}}

data: {"kind": "artifact-update", "taskId": "tsk-1", "artifact": {"parts": [{"kind": "text", "text": "hello"}]}}
```

### Python v1.0 agent (protobuf SDK, `a2a-sdk==1.0.0a0`)

Emits `StreamResponse` wrapper with `oneof` payload and SCREAMING_SNAKE_CASE enums:

```json
data: {"result": {"statusUpdate": {"taskId": "tsk-1", "status": {"state": "TASK_STATE_WORKING"}}}}

data: {"result": {"artifactUpdate": {"taskId": "tsk-1", "artifact": {"parts": [{"text": "hello"}]}}}}
```

### Differences

| Aspect | pydantic SDK (v0.3) | protobuf SDK (v1.0) |
|--------|-------------------|-------------------|
| Event wrapper | flat, `kind` discriminator | `StreamResponse.result` oneof |
| State enum | `working`, `completed` | `TASK_STATE_WORKING`, `TASK_STATE_COMPLETED` |
| Role enum | `agent`, `user` | `ROLE_AGENT`, `ROLE_USER` |
| Part format | `{"kind": "text", "text": "..."}` | `{"text": "..."}` (no `kind`) |

## Impact

1. **No single SSE format satisfies both consumers.** An agent emitting pydantic format works for the test runner but not for v1.0 agents receiving streaming callbacks in a multi-hop chain. An agent emitting protobuf format interops with v1.0 agents but the test runner may fail to parse.

2. **Streaming tests pass for the wrong reason.** The `v10-core-streaming` tests pass because token verification checks the final concatenated result, not individual SSE events. The ITK is not validating that SSE events conform to the v1.0 wire format.

3. **Agents must hide streaming capability.** When adding the Elixir agent (PR #475), we had to intentionally *not* advertise `capabilities.streaming=true` to avoid parse failures during multi-hop callbacks where different SDKs handle the downstream response.

## Suggested fixes

1. **Upgrade the ITK test runner** to use `a2a-sdk>=1.0.0a0` (protobuf SDK) — aligns with v1.0 agents natively
2. **Add format detection** in the test runner's SSE parser to handle both pydantic and protobuf event formats
3. **Separate v0.3 and v1.0 streaming tests** so each uses the matching SDK for parsing
4. **Add SSE event structure assertions** to streaming tests — currently only the final result is checked

## Reproduction

1. Run `v10-core-streaming` test (Python v1.0 + Go v1.0)
2. Capture the actual SSE events emitted by each agent
3. Compare against what `testlib.py` can parse
4. Note: tests pass but individual events may not parse correctly

## Context

Discovered while adding the Elixir v1.0 agent to the ITK (PR #475). The Elixir agent implements SSE streaming but cannot advertise it due to this format mismatch.

cc @kdziedzic70