session-hydration-csharp-parity.md

@modelcontextprotocol/server	minor

Add multi-node session hydration support, aligned with the C# SDK pattern:

• WebStandardStreamableHTTPServerTransport now accepts a sessionId constructor option. When set, the transport validates incoming mcp-session-id headers against that value and rejects re-initialization, without requiring a fresh initialize handshake on this node.
• Server.restoreInitializeState(params) restores negotiated client capabilities and version from persisted InitializeRequest params, so capability-gated server-initiated features (sampling, elicitation, roots) work on hydrated instances.

Internal refactor: the private _initialized flag is removed. Its checks are replaced by equivalent sessionId === undefined checks, so observable behavior (error codes and messages) is unchanged.