TCK sends v0.3 file part format {"file": {...}} but v1.0 Part uses flat oneof (url/raw)

## Summary

The TCK's `valid_file_message_params` fixture sends a file part in v0.3 format using a nested `file` object. The A2A v1.0 spec defines `Part` as a flat protobuf `oneof` with `text`, `raw`, `url`, and `data` fields — there is no `file` wrapper.

## Spec Reference

**File:** `specification/a2a.proto`
```protobuf
message Part {
oneof content {
string text = 1;
bytes raw = 2; // For binary file content (base64 in JSON)
string url = 3; // For file reference by URL
google.protobuf.Value data = 4; // For structured JSON data
}
string filename = 6;
string media_type = 7;
}
```

There is no `file` field. The v0.3 `FilePart` with nested `fileWithBytes`/`fileWithUri` objects was replaced by the flat `raw`/`url` fields.

## TCK Bug Location

```python
# tests/conftest.py, fixture valid_file_message_params
return {
"message": {
"parts": [
{
"file": { # ← v0.3 format, not v1.0!
"name": "test.txt",
"mediaType": "text/plain",
"fileWithUri": "https://example.com/test.txt",
}
}
]
}
}
```

## Expected v1.0 Format

```python
# File referenced by URL:
{"url": "https://example.com/test.txt", "filename": "test.txt", "mediaType": "text/plain"}

# File with inline binary content:
{"raw": "", "filename": "test.txt", "mediaType": "text/plain"}
```

## Reproduction

```bash
# Start any a2a-sdk 1.0.0a0 agent (port 9995-9998)
curl -s -X POST http://localhost:9998 \
-H "Content-Type: application/json" \
-d '{"jsonrpc":"2.0","method":"SendMessage","params":{"message":{"messageId":"msg-1","role":"ROLE_USER","parts":[{"file":{"name":"test.txt","mediaType":"text/plain","fileWithUri":"https://example.com/test.txt"}}]}},"id":"1"}'

# Expected per spec: -32602 because 'file' is not a valid Part field
# Actual (confirmed): {'error': {'code': -32602, 'data': "Message type 'lf.a2a.v1.Part' has no field named 'file'..."}}
```

The SDK correctly rejects this (the `file` field doesn't exist in v1.0 `Part`), confirming the TCK fixture is stale v0.3 format.

## Tests Affected

- `tests/conftest.py::valid_file_message_params` fixture
- `tests/optional/capabilities/test_message_send_capabilities.py::test_message_send_valid_file_part`
- `tests/optional/capabilities/test_message_send_capabilities.py::test_message_send_valid_multiple_parts`
- Any test using `valid_file_message_params`

## Fix

Replace the `file` nested format with v1.0 flat format. For a URL reference:
```python
{"url": "https://example.com/test.txt", "filename": "test.txt", "mediaType": "text/plain"}
```

## Classification

**TCK bug**: Stale v0.3 file part format in TCK fixtures. v1.0 agents correctly reject this per spec, causing TCK tests to fail on compliant implementations.

Issue nằm trong fixture `valid_file_message_params` ở `tests/conftest.py`. Kiểm tra fixture và các test sử dụng nó được liệt kê trong issue. Cập nhật fixture để sử dụng định dạng Part v1.0 phẳng với các trường `url` hoặc `raw` thay vì đối tượng `file` lồng nhau. Chạy các test bị ảnh hưởng, chẳng hạn như các test trong `tests/optional/capabilities/test_message_send_capabilities.py`, để xác minh rằng chúng pass với định dạng mới.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.