Update examples for the 2026-07-28 era

[koic]

Motivation and Context

The runnable examples drifted from the SDK they ship with:

• POST requests whose Accept header does not cover both application/json and text/event-stream are answered 406, so the cURL walkthroughs no longer worked as printed.
• MCP::Client#connect now defaults to mode: :auto, which adopts the sessionless modern lifecycle (2026-07-28) against a dual-era server; the Streamable HTTP client example requires a session and broke against its own server.
• POST responses on an established session arrive as an SSE stream, which the hand-rolled HTTP client did not parse.
• notification_tool claimed to send SSE notifications but only returned a Tool::Response.
• http_client.rb read a resource URI the server never registered and skipped notifications/initialized.

Handshake examples now offer the latest handshake protocol version (2025-11-25) instead of 2024-11-05, the Streamable HTTP client pins mode: :legacy for its session-based demo, notification_tool reports real notifications/progress events requested through _meta.progressToken, and the interactive client streams those events from the POST response so they are visible on screen.

The modern lifecycle itself had no runnable example, so a new server and client pair (modern_http_server.rb, modern_http_client.rb) shows the sessionless flow end to end: server/discover capability discovery, the per-request _meta envelope with the Mcp-Method and Mcp-Name headers (stamped by the SDK client automatically), resultType stamping, SEP-2549 cache hints, a SEP-2322 multi round-trip deploy tool resumed automatically by the client's elicitation handler, and the removal of legacy-only methods such as ping. The same server keeps serving the legacy initialize flow, routed per request by the MCP-Protocol-Version header. examples/README.md documents the pair and adds a cURL walkthrough covering the modern headers, the envelope, and the multi round-trip exchange.

How Has This Been Tested?

• ruby examples/stdio_client.rb against stdio_server.rb
• ruby examples/http_client.rb against http_server.rb
• ruby examples/streamable_http_client.rb against streamable_http_server.rb, including the progress stream
• ruby examples/modern_http_client.rb against modern_http_server.rb (discover, modern adoption, cache hints, resultType, the multi round-trip resume, ping rejection)
• The cURL walkthroughs in the banners and examples/README.md, including the requestState echo and a legacy initialize against the modern example server

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 新建 feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• 新建 and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed