Add SEP-2575 conformance tests

[Yuan325]

Add conformance tests for SEP-2575 to validate the stateless-MCP behavior. This includes conformance tests for both MCP server and client.

Server

1. Per-request _meta
   • Rejects request with missing _meta → -32602 Invalid params.
   • Rejects request with _meta missing io.modelcontextprotocol/protocolVersion.
   • Rejects request with _meta missing io.modelcontextprotocol/clientInfo.
   • Rejects request with _meta missing io.modelcontextprotocol/clientCapabilities.
2. server/discover
   • Responds to server/discover
   • Response includes required fields: supportedVersions (non-empty), capabilities, serverInfo
   • capabilities matches what the server honors on real RPC calls
3. Version negotiation
   • For a _meta.protocolVersion the server does not support, returns UnsupportedProtocolVersionError (not Invalid params and not silent rejection).
   • UnsupportedProtocolVersionError carries non-empty data.supportedVersions matching the set advertised by server/discover (no drift).
   • server/discover's supportedVersions matches the set the server actually accepts on real RPC calls (no drift).
   • Returns the error even when the unsupported version is on the server/discover request itself (and still advertises supportedVersions so the client can recover).
   • [HTTP] UnsupportedProtocolVersionError → HTTP 400 Bad Request.
   • [HTTP] Rejects request when MCP-Protocol-Version header is absent or does not match _meta.protocolVersion (HTTP 400).
4. Errors
   • MissingRequiredClientCapabilityError (-32003) carries data.requiredCapabilities listing missing keys.
   • All error responses carry the request's JSON-RPC id.
5. Removed RPCs
   • ping → -32601.
   • logging/setLevel → -32601.
   • resources/subscribe → -32601.
   • resources/unsubscribe → -32601.
6. HTTP transport
   • Unknown method → HTTP 404 Not Found + JSON-RPC -32601.
   • MissingRequiredClientCapabilityError → HTTP 400 Bad Request.

Client

1. Per-request _meta
   • Populates _meta on every request with all three required fields.
   • Accurately reports clientCapabilities per request (does not claim unsupported features).
2. server/discover
   • Should be able to call
3. Version negotiation
   • Does version negotiation correctly
   • Includes io.modelcontextprotocol/protocolVersion in _meta on every request.
   • Sends a consistent protocolVersion once chosen (does not flip between versions request-to-request).
   • [HTTP] Sends MCP-Protocol-Version header on every request, equal to _meta.protocolVersion.
4. Cancellation
   • [HTTP] Cancels by closing the stream (request).
   • [STDIO] Cancels by sending notifications/cancelled with the request id.

[Yuan325]

Updated checks scenarios according to #273