feat(langfuse): add Score event support

[danostrosky]

Summary

• Add ScoreEventBody struct and CreateScore API to the Langfuse SDK, enabling evaluation of LLM outputs via scores (e.g., user satisfaction, accuracy)
• Wire Score into eventBodyUnion serialization, trace ID resolution, and observation ID resolution
• Add unit tests and update mock

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes (including new Score union test case)
• Manual verification against a Langfuse instance with CreateScore calls

🤖 Generated with Claude Code

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