…contract (#216)

* CxODEV-1730: add StructuredErrorException + structured_error task-output contract

Adds a framework capability for workers to attach a sanitized, structured error
classification to a failed task's output:

- StructuredErrorException(code, reason, referenceError?)
- StructuredError DTO and ErrorOutput.StructuredError (omitted when null → back-compat)
- StructuredErrorSerializer: OutputKey, ToOutputData (for signal senders) and
  tolerant TryParse (consumer side); a round-trip contract test pins the shape/key
- both ExecutionManager and TypePollSpreadingExecutionManager populate structured_error
  when a StructuredErrorException is caught; plain exceptions still emit only error_message
- version bumped 3.7.3 -> 3.8.0 (additive, on the v3 line)

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>