- Added `LLMMessagesTransformFrame` to facilitate programmatically editing context in a frame-based way.

  The previous approach required the caller to directly grab a reference to the context object, grab a "snapshot" of its messages _at that point in time_, transform the messages, and then push an `LLMMessagesUpdateFrame` with the transformed messages. This approach can lead to problems: what if there had already been a change to the context queued in the pipeline? The transformed messages would simply overwrite it without consideration.
