fix: prevent ReWOO replay from mutating cached tool_calls

[Harsh-617]

Fixes #159

Use copy.copy(self.current_plan) before mutating tool_calls in both plan()
and aplan() so the original plan is preserved across replay steps.

Previously, aliasing self.current_plan caused tool_calls to shrink after
the first replay step, resulting in A->A->A execution instead of A->B->C.

Adds regression tests for both sync and async replay paths.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.09%. Comparing base (4ea91ed) to head (e37c7ab).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #160   +/-   ##
=======================================
  Coverage   90.08%   90.09%           
=======================================
  Files          19       19           
  Lines        1503     1504    +1     
=======================================
+ Hits         1354     1355    +1     
  Misses        149      149           

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[colinfrisch]

I like this and I think that we could merge it soon. I'm going to leave @sanika-n review it as she worked on ReWOO more than me. But don't hesitate if you have any questions