[perf] Drop redundant any_to_str() calls in AgentRearrange

[kyegomez]

Summary

any_to_str(current_task) is called on every agent response in _run_sequential_workflow (line 577) even though agent.run() already returns a string in the common path. Check type first to skip the conversion.

Problem

current_task = agent.run(task=self.conversation.get_str(), img=img)
current_task = any_to_str(current_task)  # Redundant when already a string

any_to_str() handles arbitrary types (dicts, lists, objects), but in practice agent.run() returns a string. The extra function call and type inspection on every agent step is wasted work.

Proposed solution

current_task = agent.run(task=self.conversation.get_str(), img=img)
if not isinstance(current_task, str):
    current_task = any_to_str(current_task)

Simple isinstance check is near-zero cost and skips the function call overhead in the common case.

Acceptance criteria

• Guard any_to_str() with isinstance(current_task, str) check
• Existing tests pass unchanged