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>

internal/readme/client/client.go

@@ -25,7 +25,11 @@ func main() {
 	}
 	defer session.Close()
 	// Call a tool on the server.
-	if res, err := session.CallTool(ctx, "greet", map[string]any{"name": "you"}, nil); err != nil {
+	params := &mcp.CallToolParams[map[string]any]{
+		Name:      "greet",
+		Arguments: map[string]any{"name": "you"},
+	}
+	if res, err := mcp.CallTool(ctx, session, params); err != nil {
 		log.Printf("CallTool failed: %v", err)
 	} else {
 		if res.IsError {