Un-deprecate/add no-arg builders

#928 deprecated no-arg builders for schema types, e.g. McpSchema.Resource

I'd like you to consider reversing that decision, and also to add no-arg versions for types without them (e.g. ProgressNotification).

I understand the motivation that builders not be left in an invalid state. It's worth noting that it might achieve that aim currently (I didn't audit all the classes), but if the schema ever has any fields which are mutually exclusive or conditionally required, then using constructors will only offer partial protection.

The problem is that it makes it undermines one of the main advantages of the builder pattern, which is to make construction clearer.

Here was my attempt to write a CreateMessageRequest with only required properties:

var request = McpSchema.CreateMessageRequest.builder(
        List.of(McpSchema.SamplingMessage.builder(
            McpSchema.Role.USER,
            McpSchema.TextContent.builder("Test Sampling Message").build()).build()
        ),
        50
    )
    .build();

Maybe there's a better way to format/indent this, but I tried several variations and I thought this was the best one.

Compare that with the same thing constructed with named properties

var request = McpSchema.CreateMessageRequest.builder()
    .messages(List.of(
        McpSchema.SamplingMessage.builder()
            .role(McpSchema.Role.USER)
            .content(McpSchema.TextContent.builder("Test Sampling Message").build())
            .build()
    ))
    .maxTokens(50)
    .build();

It's also worth noting that some builders have APIs like ModelPreferences#addHint. If that pattern were applied consistently, it could be simplified a little more by replacing messages(List.of( with addMessage(

Beyond readability

I'm writing a framework and builders enforcing all required params at once makes them inflexible for some possible API designs.

For example, my framework automatically manages progress tokens. I considered a design such as

void sendProgress(Consumer<McpSchema.ProgressNotification.Builder> consumer);

// an example caller. mcp creates the builder and applies the token
mcp.sendProgress(p -> p.progress(5).total(10));

However, it's not possible for the framework to implement this with the current builder methods. The user of the framework knows the progress value, the framework itself knows the progress token, but the builder expects both at once.

So the builder has to be worked around, for example to this

void sendProgress(double progress, Consumer<McpSchema.ProgressNotification.Builder> consumer);
// an example caller
mcp.sendProgress(5, p -> p.total(10));

Start with the McpSchema builder entry points named here, including CreateMessageRequest, SamplingMessage, TextContent, ProgressNotification, and ModelPreferences. Compare the required-argument and no-argument APIs, then check whether schema values can be supplied incrementally, including framework-managed progress tokens; done means the builder approach is consistent across the requested types.