fix example split handling [LSDK-424]

[Shamik Karkhanis (shamikkarkhanis)]

Summary

• create examples using the native LangSmith split request field
• filter example listings through the API native splits query
• keep user metadata independent from split membership
• display API-provided dataset split membership in pretty output

Functional reproduction

Used one temporary production dataset containing:

• a native test example whose user metadata had split=metadata-only
• a native train example
• a base example whose user metadata had split=test

Before: example list --split test returned the base metadata-only match and omitted the native test example.

After: the same command returned only the native test example, and pretty output showed its API-provided test membership. The temporary dataset was deleted afterward.

Test plan

• go test ./...
• go vet ./...
• live create/list/filter workflow against a temporary LangSmith dataset

Linear: https://linear.app/langchain/issue/LSDK-424