This PR introduces the SessionMessage type to encapsulate JSONRPC messages with metadata. This change enables clean separation between protocol messages and transport concerns.

Problem

Previously, memory streams propagated raw JSONRPCMessage objects, which made it difficult to pass transport-specific metadata (like resumption tokens or request correlation IDs) without violating the JSONRPC spec.

Proposed solution

Introduced a new SessionMessage type that wraps JSONRPCMessage with optional metadata:

  class SessionMessage:
      message: JSONRPCMessage
      metadata: MessageMetadata = None

With specialized metadata types:

• ClientMessageMetadata: Contains resumption_token for client-side resumability and will contain additional fields needed for resumability
• ServerMessageMetadata: Contains related_request_id for server-side request correlation

Changes

• All memory streams now use SessionMessage instead of raw JSONRPCMessage
• Session classes extract the underlying JSONRPC message for protocol operations
• Transport implementations now can use metadata for transport-specific features:

NOTE: stacked on top of #587