MCP servers now work across pods via session ID token

Minor changes

• 37aafd3 feat(mcp): stateless and multi-pod server support — carry $session_id and the client identity (harness) across pods via a self-encoded Mcp-Session-Id token minted at initialize and replayed on every request. Auto-wired on the instrument() FastMCP path (stateless_http=True); custom PostHogMCP dispatchers add PostHogMcpStatelessSessionMiddleware and read get_mcp_session(). — Thanks @gesh!