Streaming Error Span schema

[nichwch]

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Adds validated_chunk field to ValidationResult, and error_spans to FailResult. Error spans track the specific character index ranges that are causing validation failure.

Additionally, a helper method that converts error spans in a fail result (which are indexed relative to the validated_chunk) into error spans that are relative to the entire LLM output

[notion-workspace[bot]]

Rewrite chunk emission to text spans and validated spans on API