Store trace context in emit_event, and link it in 'await_event'

[Aaron1011]

This should result in Sentry displaying a link from the task execution trace (where 'await_event' is called) back to the trace that performed the 'emit_event' call

Note that this is a breaking change, as we now wrap the user's payload in a struct when reading/writing to the database. Going forward, we'll be able to add new (optional) fields to this wrapper struct without breaking existing durable deployments

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Note

Introduces trace-aware event payloads and links traces across emit/await.

• Adds EventPayloadWrapper and wraps all event payloads in client.emit_event[_with], injecting trace_context when telemetry is enabled; DB now stores the wrapper instead of raw payload (breaking change)
• Updates TaskContext.await_event to decode the wrapper and (telemetry) add a span link to the emitter trace; caches the wrapped JSON
• Refactors TaskContext.emit_event to delegate to client and adds TaskError::EmitEventFailed (with serialization for persistence)
• Adjusts tests to emit wrapper-shaped JSON directly when calling SQL

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