Provide means to access Messages for ChatModelAgentMiddleware

Since `ProcessState` is `compose` layer API, we prefer to hide it from ChatModelAgent users. To fully replace it's functionality, we need to provide first-class support to 'Get' current message list in ChatModelAgentMiddleware, other than from Before/AfterModelRewriteState.

We already have concrete user request for this feature.

Start by tracing ChatModelAgentMiddleware and the BeforeModelRewriteState and AfterModelRewriteState APIs to understand how current messages are exposed today. Compare that with the ProcessState compose-layer API and identify the middleware-facing access point needed to replace it. Done means ChatModelAgent users can retrieve the current message list without depending on ProcessState or rewrite-state details.

Written by the indexing model from the issue text.