SSE GET session crashes on 404 instead of treating it as "SSE not supported" #589

• Target: client
• Component: sdk-client
• Scenarios: tools_call, auth/scope-step-up, auth/scope-retry-limit

Root cause

After sending notifications/initialized, StreamableHttpClientTransport.startSseSession opens a GET
SSE stream. The conformance runner returns 404 for GET. The transport only handles 405 (Method Not Allowed) as "server
doesn't support SSE" but does not handle 404. The Ktor SSE client throws
SSEClientException("Expected status code 200 but was 404"), which propagates up and crashes the client during
initialization