fix: Remove in-memory _messages field on Agent

[mattzh72]

Description

Before, we have in-memory stores tracking the in context messages and metadata associated with those messages.

This would require careful calls of load and save for the agent before and after manipulating the in context messages. We change this model to instead track this in the database, and rely on the transactional nature of the database to keep consistent in-context memory state.

As a result:

• We can guarantee that the state is consistent
• Reduce a large amount of code dedicated to checks, and instead rely on the ORM
• Reduce our dependence on the load_agent function and instead use lightweight db R/W calls

Some small clean up work to merge the two summarizer tests into the integration summarizer test.

Testing

Rely on existing testing since this should not change behavior, and also add new manager tests.