internal/mcp: implement HTTP SSE transport

Add support for SSE transport, as defined by version 2024-11-05 and
2025-03-26 of the MCP spec.

In order to achieve this, the jsonrpc2 stream must be abstracted so that
we can implement it across multiple HTTP requests. This required some
refactoring of the jsonrpc2_v2 package, so that we can avoid the
Dialer/Binder interfaces and construct the Connection directly from its
constituent primitives.

Change-Id: Id627b5b5a474d92f2c613cfad904d14611e7c768
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/util.go

@@ -4,6 +4,8 @@
 
 package mcp
 
+import "crypto/rand"
+
 func assert(cond bool, msg string) {
 	if !cond {
 		panic(msg)
@@ -14,3 +16,17 @@ func is[T any](v any) bool {
 	_, ok := v.(T)
 	return ok
 }
+
+// Copied from crypto/rand.
+// TODO: once 1.24 is assured, just use crypto/rand.
+const base32alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
+
+func randText() string {
+	// ⌈log₃₂ 2¹²⁸⌉ = 26 chars
+	src := make([]byte, 26)
+	rand.Read(src)
+	for i := range src {
+		src[i] = base32alphabet[src[i]%32]
+	}
+	return string(src)
+}