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/transport.go

@@ -200,7 +200,7 @@ func (s *loggingStream) Read(ctx context.Context) (jsonrpc2.Message, int64, erro
 		if err != nil {
 			fmt.Fprintf(s.w, "LoggingTransport: failed to marshal: %v", err)
 		}
-		fmt.Fprintf(s.w, "read: %s", string(data))
+		fmt.Fprintf(s.w, "read: %s\n", string(data))
 	}
 	return msg, n, err
 }
@@ -215,7 +215,7 @@ func (s *loggingStream) Write(ctx context.Context, msg jsonrpc2.Message) (int64,
 		if err != nil {
 			fmt.Fprintf(s.w, "LoggingTransport: failed to marshal: %v", err)
 		}
-		fmt.Fprintf(s.w, "write: %s", string(data))
+		fmt.Fprintf(s.w, "write: %s\n", string(data))
 	}
 	return n, err
 }