CrewAgentExecutor does not reset messages and iterations between task executions

[Killian-fal]

Fixes #4389

Description :
reset state before agent execution to prevent stale data

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Note

Low Risk
Small, localized behavioral change with added test coverage; primary risk is breaking any callers that implicitly relied on cross-run message/iteration persistence.

Overview
Fixes stale state leakage when a CrewAgentExecutor instance is reused by clearing messages and resetting iterations to 0 at the start of both invoke() and ainvoke().

Updates async executor tests to assert stale state is cleared before execution, and adds a new test ensuring sequential invoke() calls on the same executor start with isolated message histories and do not carry over iteration counts.

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[lorenzejay]

awesome ! let me just double check one thing, but this looks good to me

[lorenzejay]

lovely!