internal/mcp: update protocol versions

Update protocol versions to reflect that 2025-03-26 is supported, and
add server logic to negotiate the supported version with the client.

Update the client to set the protocol version, fixing what was probably
a spec violation.

+tests

Change-Id: I564b591b3daf018dd54f5c7e5d8f2d2b1e4ff502
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682516
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

mcp/conformance_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build (go1.24 && goexperiment.synctest) || go1.25
 
 package mcp
 
@@ -85,7 +85,7 @@ func TestServerConformance(t *testing.T) {
 			// By comparison, gopls has a complicated framework based on progress
 			// reporting and careful accounting to detect when all 'expected' work
 			// on the server is complete.
-			synctest.Run(func() { runServerTest(t, test) })
+			runSyncTest(t, func(t *testing.T) { runServerTest(t, test) })
 
 			// TODO: in 1.25, use the following instead:
 			// synctest.Test(t, func(t *testing.T) {