Revert "Revert "Enable parallel tool calling with new_task isolation safeguards""

[mrubens]

Reverts #11004

[roomote]

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This PR re-enables parallel tool calling with proper safeguards for new_task isolation. The implementation correctly:

• Enforces that tools appearing after new_task in parallel calls are truncated with error results
• Saves the assistant message to API history before tool execution to maintain correct message ordering
• Includes comprehensive test coverage for the isolation enforcement logic

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}