[Feat]: Add `min_items = 1` constraint to `Artifact` in protobuf schema

### Is your feature request related to a problem? Please describe.

The [A2A Protocol Specification (Section 4.1.7)](https://a2a-protocol.org/latest/specification/#artifact) states that an Artifact's `parts` array "Must contain at least one part."

However, this constraint is not reflected in the protobuf schema definition at `specification/a2a.proto`.

### Describe the solution you'd like

### Current Schema

**specification/a2a.proto**

```proto
message Artifact {
// Unique identifier (e.g. UUID) for the artifact. It must be at least unique
// within a task.
string artifact_id = 1 [(google.api.field_behavior) = REQUIRED];
// A human readable name for the artifact.
string name = 3;
// A human readable description of the artifact, optional.
string description = 4;
// The content of the artifact. Must contain at least one part.
repeated Part parts = 5 [(google.api.field_behavior) = REQUIRED];
// Optional metadata included with the artifact.
google.protobuf.Struct metadata = 6;
// The URIs of extensions that are present or contributed to this Artifact.
repeated string extensions = 7;
}
```

**specification/buf.yaml**

```yaml
version: v2
deps:
# Common Protobuf types.
- buf.build/googleapis/googleapis
```

### Proposed Change

**specification/a2a.proto**

```diff
+ import "buf/validate/validate.proto";

message Artifact {
// Unique identifier (e.g. UUID) for the artifact. It must be at least unique
// within a task.
string artifact_id = 1 [(google.api.field_behavior) = REQUIRED];
// A human readable name for the artifact.
string name = 3;
// A human readable description of the artifact, optional.
string description = 4;
// The content of the artifact. Must contain at least one part.
- repeated Part parts = 5 [(google.api.field_behavior) = REQUIRED];
+ repeated Part parts = 5 [
+ (google.api.field_behavior) = REQUIRED,
+ (buf.validate.field).repeated.min_items = 1
+ ];
// Optional metadata included with the artifact.
google.protobuf.Struct metadata = 6;
// The URIs of extensions that are present or contributed to this Artifact.
repeated string extensions = 7;
}
```

**specification/buf.yaml**

```diff
version: v2
deps:
# Common Protobuf types.
- buf.build/googleapis/googleapis
+ - buf.build/bufbuild/protovalidate
```

## Why This Matters

SDKs that generate models from predefined schema in A2A repository (e.g., https://github.com/a2aproject/a2a-python using `datamodel-codegen`) do not automatically enforce the non-empty constraint because the schema lacks `min_items`. This means invalid `Artifact` objects with empty parts can be created without any validation error.

- The Java SDK already enforces this at the code level but this should be driven by the schema itself.
- https://github.com/a2aproject/a2a-java/blob/v0.3.3.Final/spec/src/main/java/io/a2a/spec/Artifact.java
- The Python SDK applied a manual fix in below but since `types.py` is auto-generated from this schema, the fix will be overwritten on the next regeneration.
- https://github.com/a2aproject/a2a-python/issues/670
- https://github.com/a2aproject/a2a-python/pull/671

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct