internal/mcp: generic handling of CallToolParams

Make Tool APIs future-proof by pushing generics into CallToolParams, and
using CallToolParams throughout. This has several advantages:

- We no longer need the awkward CallToolOptions type, which is almost
  but not quite like CallToolParams.
- NewTool no longer needs to define its own handler type: it accepts a
  ToolHandler[TArgs]. This also means its signature is future-proof.

Additionally, provide a `CallTool` helper (the analog of NewTool) to
serve as a more strongly typed variant of `ClientSession.CallTool`.

+ design, protocol generation

Change-Id: I80172a25bc50734dae84a541aa4776ed9161694e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676495
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/cmd_test.go

@@ -53,7 +53,10 @@ func TestCmdTransport(t *testing.T) {
 	if err != nil {
 		log.Fatal(err)
 	}
-	got, err := session.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil)
+	got, err := mcp.CallTool(ctx, session, &mcp.CallToolParams[map[string]any]{
+		Name:      "greet",
+		Arguments: map[string]any{"name": "user"},
+	})
 	if err != nil {
 		log.Fatal(err)
 	}