internal/mcp: support prompts

Add support for 'prompts/list' and 'prompts/get', and registering
prompts on the server using AddPrompts. Add a 'MakePrompt' helper to
construct prompts from a prompt handler using reflection.

Change-Id: I479cb9c9b99313cca99640c1f5ea1939363759e6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/util/util.go

@@ -0,0 +1,36 @@
+// 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 util
+
+import (
+	"cmp"
+	"iter"
+	"slices"
+)
+
+// Helpers below are copied from gopls' moremaps package.
+
+// sorted returns an iterator over the entries of m in key order.
+func Sorted[M ~map[K]V, K cmp.Ordered, V any](m M) iter.Seq2[K, V] {
+	// TODO(adonovan): use maps.Sorted if proposal #68598 is accepted.
+	return func(yield func(K, V) bool) {
+		keys := KeySlice(m)
+		slices.Sort(keys)
+		for _, k := range keys {
+			if !yield(k, m[k]) {
+				break
+			}
+		}
+	}
+}
+
+// keySlice returns the keys of the map M, like slices.Collect(maps.Keys(m)).
+func KeySlice[M ~map[K]V, K comparable, V any](m M) []K {
+	r := make([]K, 0, len(m))
+	for k := range m {
+		r = append(r, k)
+	}
+	return r
+}