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/features_test.go

@@ -18,9 +18,11 @@ type SayHiParams struct {
 	Name string `json:"name"`
 }
 
-func SayHi(ctx context.Context, cc *ServerSession, params *SayHiParams) ([]*Content, error) {
-	return []*Content{
-		NewTextContent("Hi " + params.Name),
+func SayHi(ctx context.Context, cc *ServerSession, params *CallToolParams[SayHiParams]) (*CallToolResult, error) {
+	return &CallToolResult{
+		Content: []*Content{
+			NewTextContent("Hi " + params.Name),
+		},
 	}, nil
 }