internal/mcp: sampling

Implement sampling (the CreateMessage method).

Change-Id: Ie44a671dd62831122b25cd7386b5bfcdece7ff60
Reviewed-on: https://go-review.googlesource.com/c/tools/+/673176
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

mcp/shared.go

@@ -134,3 +134,16 @@ func sessionMethod[S ClientSession | ServerSession, P, R any](f func(*S, context
 		return f(sess, ctx, p)
 	}
 }
+
+// Error codes
+const (
+	// The error code to return when a resource isn't found.
+	// See https://modelcontextprotocol.io/specification/2025-03-26/server/resources#error-handling
+	// However, the code they chose in in the wrong space
+	// (see https://github.com/modelcontextprotocol/modelcontextprotocol/issues/509).
+	// so we pick a different one, arbirarily for now (until they fix it).
+	// The immediate problem is that jsonprc2 defines -32002 as "server closing".
+	CodeResourceNotFound = -31002
+	// The error code if the method exists and was called properly, but the peer does not support it.
+	CodeUnsupportedMethod = -31001
+)