Feature request: Caching the MCP server process across invocations

[clareliguori]

Today, the library always starts a fresh MCP server process on each invocation. Certain MCP servers may be able to be re-used across Lambda function invocations, and be successfully frozen and thawed between invocations to improve "warm" performance. There are security concerns around server re-use, like inadvertently sharing any in-memory state that the MCP server may save across requests. But, users can be responsible for determining whether the MCP server is safe to be re-used across function callers.

See #361 for more details