internal/mcp: middleware on the sending side

Implement sending middleware.

Sending middleware is useful for adding tracing, progress tokens,
and so on.

Change-Id: I0acab2550f0212d549c4adbcf1ebf425a9486df6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676355
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/example_progress_test.go

@@ -0,0 +1,29 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp_test
+
+import (
+	"context"
+	"sync/atomic"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+var nextProgressToken atomic.Int64
+
+// This middleware function adds a progress token to every outgoing request
+// from the client.
+func Example_progressMiddleware() {
+	c := mcp.NewClient("test", "v1", nil)
+	c.AddSendingMiddleware(addProgressToken[*mcp.ClientSession])
+	_ = c
+}
+
+func addProgressToken[S mcp.Session](h mcp.MethodHandler[S]) mcp.MethodHandler[S] {
+	return func(ctx context.Context, s S, method string, params mcp.Params) (result mcp.Result, err error) {
+		params.GetMeta().ProgressToken = nextProgressToken.Add(1)
+		return h(ctx, s, method, params)
+	}
+}