fix: support freeform stopReason in sampling

[LucaButBoring]

Supports freeform stop reasons in sampling responses by mapping them to a new UNKNOWN value.

Motivation and Context

Previously, sampling response parsing would fail if a stopReason was provided besides endTurn/stopSequence/maxTokens. This fixes that to map the arbitrary values to UNKNOWN instead.

Spec: https://github.com/modelcontextprotocol/modelcontextprotocol/blob/f5ccad944fdf2b7d9cc70cf817f66ca5a8aa03a4/schema/2024-11-05/schema.ts#L807

How Has This Been Tested?

Unit tests

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Closes #328

[tzolov]

@LucaButBoring, thanks for addressing this.
The UNKNOWN approach masks the original stop reason. I'm not sure how important the exact stop-reason is for downstream applications?
A better approach would be relaxing the stop reason type to String, but this would be a breaking change. What do you think?

[LucaButBoring]

The UNKNOWN approach masks the original stop reason. I'm not sure how important the exact stop-reason is for downstream applications?
A better approach would be relaxing the stop reason type to String, but this would be a breaking change. What do you think?

Yes, I was thinking about this - the reason I added UNKNOWN was to avoid needing to do this. I think there are better ways to model StopReason in general, but didn't want to make a breaking change without a good reason to do so. Also, I did want to keep the well-modeled known stop reasons, though it's not strictly necessary to do so. Given that we can't really have a union of enums and a string like TS/Python, I'd probably want to lean towards a Kotlin-esque approach and do this if we accepted a breaking change:

public sealed class StopReason permits EndTurn, StopSequence, MaxTokens, Unknown {
    private String value;

    public StopReason(String value) {
        this.value = value;
    }

    public class EndTurn : StopReason {
        public EndTurn() {
            super("endTurn");
        }
    }

    public class StopSequence : StopReason {
        public EndTurn() {
            super("stopSequence");
        }
    }

    public class MaxTokens : StopReason {
        public MaxTokens() {
            super("maxTokens");
        }
    }

    public class Unknown : StopReason {
        public Unknown(String value) {
            super(value);
        }

        public String getValue() {
            return value;
        }
    }
}

That way consumers could handle it like so:

if (stopReason instanceof EndTurn) {
    // ...
} /* other cases */ else if (stopReason instanceof Unknown unknownStopReason) {
    log.info("Sampling stopped due to unhandled stop reason: {}", unknownStopReason)
}

I'll defer to you, though - if we're willing to make a breaking change for this and prefer that it just become a String, we can do that, instead.