fix: separate sessionNotFoundMessage into its own const block (SA9004)

Author: Copilot

internal/mcp/http_transport.go

@@ -32,9 +32,10 @@ const (
 	HTTPTransportSSE HTTPTransportType = "sse"
 	// HTTPTransportPlainJSON uses plain JSON-RPC 2.0 over HTTP POST (non-standard)
 	HTTPTransportPlainJSON HTTPTransportType = "plain-json"
-	sessionNotFoundMessage                   = "session not found"
 )
 
+const sessionNotFoundMessage = "session not found"
+
 // MCPProtocolVersion is the MCP protocol version used only by the plain JSON-RPC
 // fallback path in this package. Streamable and SSE transports are SDK-managed
 // and negotiate protocol versions internally.