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Make trace IDs parseable from hex and span contexts JSON-friendly

Make trace IDs parseable from hex and span contexts JSON-friendly: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Our tracing core can already render a TraceID to its 32-character hex string (via SpanContext.TraceIDString() ), but there's no way to go the other…

The task

Our tracing core can already render a `TraceID` to its 32-character hex string (via `SpanContext.TraceIDString()`), but there's no way to go the other direction, and dumping a `SpanContext` to JSON produces something unreadable.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.