mcp: change tool handler design (#325)

Change the design of tool handlers by removing genericity from the common path.
- `ToolHandler` gets the args as a `json.RawMessage`. Using Server.AddTool does no unmarshaling or schema validation.
- `ToolHandlerFor`, installed with `AddTool`, are the only generic pieces.

TODO:
uncomment and fix some tests

mcp/features_test.go

@@ -5,7 +5,6 @@
 package mcp
 
 import (
-	"context"
 	"slices"
 	"testing"
 
@@ -18,14 +17,6 @@ type SayHiParams struct {
 	Name string `json:"name"`
 }
 
-func SayHi(ctx context.Context, cc *ServerSession, params *CallToolParamsFor[SayHiParams]) (*CallToolResultFor[any], error) {
-	return &CallToolResultFor[any]{
-		Content: []Content{
-			&TextContent{Text: "Hi " + params.Name},
-		},
-	}, nil
-}
-
 func TestFeatureSetOrder(t *testing.T) {
 	toolA := &Tool{Name: "apple", Description: "apple tool"}
 	toolB := &Tool{Name: "banana", Description: "banana tool"}