add support for serverless deployments and session state persis…

[ibakaidov]

• add a pluggable ServerSessionStateStore interface plus an in-memory implementation for persistence
• update streamable handler/transport to load, save, and clear session state so serverless deployments can resume conversations
• document the serverless workflow and add coverage proving state survives between independent handler instances

Testing

• go test ./...

[findleyr]

Hi, I don't think we're likely to accept anything implementing session state persistence until ongoing discussions like SEP-1442 are resolved.

[maciej-kisiel]

The concept of transport level sessions is being removed from the specification for now: modelcontextprotocol/modelcontextprotocol#2567. This means that we will not be supporting session-related extension points on the SDK level at this point.