Mark SSE transport as deprecated

The HTTP+SSE transport was replaced by Streamable HTTP in protocol revision 2025-03-26 (spec PR #206). The spec now refers to it as the "deprecated HTTP+SSE transport" and only documents it for backwards compatibility.

The Python SDK still exposes the SSE transport with no indication that it is deprecated.

Proposal

Add warnings.warn(..., DeprecationWarning, stacklevel=2) and @deprecated docstring markers to:

• sse_client
• SseServerTransport
• MCPServer.run(transport="sse") / run_sse_async() / sse_app()

The warning should point users to streamable-http / StreamableHTTPTransport instead.

Prior art

The TypeScript SDK already marks SSEClientTransport as @deprecated with a note to prefer StreamableHTTPClientTransport.

Open questions

• Should we remove SSE entirely in v2, or deprecate now and remove in a later major?
• The 2024-11-05 spec is still technically valid — clients connecting to old servers may still need the SSE client. The deprecation message should acknowledge this (same as the TS SDK wording does).

_{AI Disclaimer}

Commencez par src/mcp/client/sse.py, src/mcp/server/sse.py et les méthodes de MCPServer nommées dans l’issue dans src/mcp/server/mcpserver/server.py. Comparez la formulation de la dépréciation de SSE du TypeScript SDK, puis vérifiez que les APIs listées émettent DeprecationWarning et que leur documentation oriente les utilisateurs vers Streamable HTTP tout en reconnaissant la compatibilité avec les serveurs legacy.