fix: Don't autogenerate message id (0.3 TCK has a test for a null one)

[kabir]

No description provided.

[gemini-code-assist]

Code Review

This pull request refactors several core specification classes in the compat-0.3 module (such as Message_v0_3, Task_v0_3, and various security schemes and parts) from standard classes to Java records. This change updates all getter method invocations to record accessor methods across clients, transports, mappers, and tests. Additionally, a new test infrastructure module tests/server-common is introduced. The review feedback highlights potential NullPointerException risks and validation failures if messageId is null, recommending that messageId be marked as @Nullable in Message_v0_3 and properly null-checked before being set in the protobuf builder in ProtoUtils_v0_3.

[gemini-code-assist]

Since the 0.3 TCK has a test for a null messageId (as mentioned in the PR title), messageId should be annotated with @Nullable and the non-null assertion Assert.checkNotNullParam("messageId", messageId); should be removed from the constructor to prevent unexpected validation failures.

public record Message_v0_3(
        Role role,
        List<Part_v0_3<?>> parts,
        @Nullable String messageId,
        @Nullable String contextId,
        @Nullable String taskId,
        @Nullable List<String> referenceTaskIds,
        @Nullable Map<String, Object> metadata,
        @Nullable List<String> extensions,
        String kind
) implements EventKind_v0_3, StreamingEventKind_v0_3 {

    public static final String KIND = "message";

    public Message_v0_3 {
        Assert.checkNotNullParam("role", role);
        Assert.checkNotNullParam("parts", parts);
        parts = List.copyOf(parts);
        if (parts.isEmpty()) {
            throw new IllegalArgumentException("Parts cannot be empty");
        }
        if (kind == null) {
            kind = KIND;
        }
        if (!kind.equals(KIND)) {
            throw new IllegalArgumentException("Invalid Message");
        }
    }

[gemini-code-assist]

If messageId can be null, calling builder.setMessageId(message.messageId()) directly will throw a NullPointerException because protobuf's generated setters do not accept null values. A null check should be added here, similar to contextId and taskId.

            if (message.messageId() != null) {
                builder.setMessageId(message.messageId());
            }