Implement SEP-1699: Support SSE Polling via Server-Side Disconnect

[felixweinberger]

This is a tracking issue for implementation of SEP-1699.

Summary

This SEP proposes changes to the Streamable HTTP transport to mitigate issues with long-running connections and improve resumability by allowing servers to disconnect at will after sending an SSE event ID. The Python SDK needs to implement server-side support for immediately sending events with IDs and empty data strings to prime clients for reconnection, enable servers to close connections at any time after sending event IDs, implement retry field support to prevent excessive reconnection attempts, and ensure clients handle disconnections as network failures with proper reconnection logic using Last-Event-ID headers for seamless resumption.

Related Issues & PRs

• Implementation PRs: n/a
• Related PRs: #612
• Related Issues: #1437, #1227, #1450