feat: pre invocation prompt hook

[cape2333]

Related Issue Fixes #3487

Summary

• Add a MessageMiddleware base class and middleware pipeline to ChatAgent, enabling pre-invocation
  (request) and post-invocation (response) hooks for intercepting and transforming messages sent to /
  received from the model backend
• Introduce MiddlewareContext (carries model type, agent ID, iteration info) and MiddlewareError for
  structured error propagation
• Integrate middleware execution into both sync (_get_model_response) and async (_aget_model_response)
  code paths
• Add an end-to-end example demonstrating user-profile injection and sensitive-data scrubbing middleware
• Add unit tests covering request/response modification, middleware ordering, async path, and error
  wrapping

Description

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• I have updated the tests accordingly (required for a bug fix or a new feature)
• I have updated the documentation if needed
• I have added examples if this is a new feature

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[waleedalzarooni]

Great job @cape2333,

A couple of quick comments from me!