fix(server): merge Task history in ResultManager instead of overwriting (#524)

# Description

## What

`ResultManager.processEvent('task')` merges the incoming Task with the
persisted
Task (preserving history + artifacts) instead of wholesale replacement.

## Why

Multi-turn conversations: after INPUT_REQUIRED → follow-up, the executor
often
publishes a fresh Task event without history. Wholesale replacement lost
the
entire conversation history.

Closes #532

Author: JakubWorek