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/streamable_test.go

@@ -122,7 +122,7 @@ func TestStreamableServerTransport(t *testing.T) {
 			Resources: &resourceCapabilities{ListChanged: true},
 			Tools:     &toolCapabilities{ListChanged: true},
 		},
-		ProtocolVersion: "2024-11-05",
+		ProtocolVersion: "2025-03-26",
 		ServerInfo:      &implementation{Name: "testServer", Version: "v1.0.0"},
 	}, nil)
 	initializedMsg := req(0, "initialized", &InitializedParams{})