fix(server-utils): Record LangGraph span I/O for non-MessagesAnnotation state

[zkasuran]

instrumentLangGraph recorded span input and output only when the graph state uses MessagesAnnotation. The input read did args[0].messages ?? [] and the output helper returned early when result.messages was not an array, so a graph on a custom state annotation produced an empty gen_ai.input.messages and no gen_ai.response.text. No error, just an empty invoke_agent span in the AI Agents view.

This keeps the MessagesAnnotation path unchanged. When there is no messages array, it serializes the whole input and output state and records it, wrapped as a single { role, content } message so the attribute stays a valid chat array (the convention extractLLMRequestAttributes already uses in this package). A null input (the resume case) records nothing rather than a misleading empty array. Serialization uses core's circular-safe stringify so an unusual state object cannot throw inside the span callback.

Fixes #19628

AI assistance (Claude, Anthropic) was used in developing this change. The design, review and verification were done by the author. Verified locally: the @sentry/server-utils vitest suite (335 to 339 passing), the new tests failing before and passing after the fix, oxlint --type-aware, oxfmt --check, tsc on the changed files, plus a real Google Gemini LangGraph run showing the span input and output empty before and populated after.

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