internal/mcp: add JSON conformance tests, and fix some bugs

Add the beginnings of some testdata-driven conformance tests that check
JSON-level compatibility of SDK messages, and fix some bugs in our JSON
message handling:

- "params" can be missing
- "_meta" should not be null
- lists of tools/prompts/resources should not be null

In order to implement this, we need to be able to know when the server
is done processing messages. In general, this is a hard problem; for
gopls we do this with a complicated system of progress reporting and
accounting. However, the new synctest package makes this trivial, as we
can call synctest.Wait to detect when the server is idle. Therefore,
this test requires 1.25 (it uses synctest.Test, added in 1.25, and so
can't run at 1.24 with GOEXPERIMENT=synctest).

Change-Id: I97fa28b56868340ece266a58e1eb3c3bf8cfe0e9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676538
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

mcp/server.go

@@ -189,6 +189,7 @@ func (s *Server) listPrompts(_ context.Context, _ *ServerSession, params *ListPr
 	}
 	res := new(ListPromptsResult)
 	res.NextCursor = nextCursor
+	res.Prompts = []*Prompt{} // avoid JSON null
 	for _, p := range prompts {
 		res.Prompts = append(res.Prompts, p.Prompt)
 	}
@@ -219,6 +220,7 @@ func (s *Server) listTools(_ context.Context, _ *ServerSession, params *ListTool
 	}
 	res := new(ListToolsResult)
 	res.NextCursor = nextCursor
+	res.Tools = []*Tool{} // avoid JSON null
 	for _, t := range tools {
 		res.Tools = append(res.Tools, t.Tool)
 	}
@@ -248,6 +250,7 @@ func (s *Server) listResources(_ context.Context, _ *ServerSession, params *List
 	}
 	res := new(ListResourcesResult)
 	res.NextCursor = nextCursor
+	res.Resources = []*Resource{} // avoid JSON null
 	for _, r := range resources {
 		res.Resources = append(res.Resources, r.Resource)
 	}
@@ -296,6 +299,10 @@ func (s *Server) readResource(ctx context.Context, ss *ServerSession, params *Re
 // are always caught. Go 1.24 and above also protects against symlink-based attacks,
 // where symlinks under dir lead out of the tree.
 func (s *Server) FileResourceHandler(dir string) ResourceHandler {
+	return fileResourceHandler(dir)
+}
+
+func fileResourceHandler(dir string) ResourceHandler {
 	// Convert dir to an absolute path.
 	dirFilepath, err := filepath.Abs(dir)
 	if err != nil {