Feat/async wat refresh sample

[guilhermegreco]

Amazon Bedrock AgentCore Samples Pull Request

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Issue number: #1395

Concise description of the PR

Changes to add async WAT refresh sample under 03-AgentCore-identity, because long-running threaded agents lose the WAT after JWT expiration, causing orphaned workload identities. This sample demonstrates a companion library that propagates WAT to threads and coordinates refresh with the client.

User experience

Before: Developers running async agents with JWT inbound auth get orphaned workload identities when the WAT expires mid-task.
After: Developers use @with_wat_refresh decorator and ThreadTaskManager to automatically pause, refresh, and retry — no orphans, user binding preserved.

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