Is your feature request related to a problem? Please describe.
When consuming update (and standalone activity) results through the SDK's high-level types, callers like the CLI
cannot access the full underlying proto data:

• Errors: PollWorkflowUpdate returns a bare error. The underlying temporalError preserves the original
  *failurepb.Failure proto (including stack trace, structured error chain, encoded details), but failure() is
  unexported. There's no public way to extract the full structured failure for display.
• Results: PollWorkflowUpdate returns EncodedValue, which only exposes Get(valuePtr) — it decodes the payload into
  a Go type, discarding the raw *commonpb.Payload. Callers that need the raw payload (e.g., to render with or
  without JSON shorthand) have no way to access it.

This affects CLI issue temporalio/cli#952.

Describe the solution you'd like
Add public accessors to the existing SDK types:

1. A public Failure() *failurepb.Failure method on temporalError (or a public interface that error types
   implement), so callers can access the full proto failure including stack trace, error chain, and encoded details.
2. A way to get the raw *commonpb.Payload from EncodedValue without going through data conversion — e.g., a
   Payload() *commonpb.Payload accessor or similar.

This keeps the high-level types as the primary API while making them fully inspectable for advanced callers.