Add file-based ChatMemoryStore implementation

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Expected Behavior

I've seen other mcp memory stores in python and npm that have a local file-based memory store that is useful for testing and for local development as well as possible other uses. I'm thinking we store memory as a JSON object and have it auto-configure in the application.properties or application.yml.

Current Behavior

Currently we only have in-memory, jdbc, neo4j, and possibly soon a mongo memory. Having the option to supply a json file isn't included directly.

Context

We could extend the in-memory chat memory to allow for a file to be picked up when it initializes, but it would be nice to have a way to combine the two in a local file system method that uses auto-config.

Start by reviewing the existing in-memory, JDBC, Neo4j, and possible Mongo chat memory store implementations, then inspect how application.properties or application.yml configures them. Define the file-based store's JSON persistence, initialization behavior, interaction with in-memory memory, and auto-configuration before implementing it; done includes coverage for persistence and configuration.