refactor: replace mcp-go with the official MCP Go SDK (#223)

## Summary

Replace `github.com/mark3labs/mcp-go` with the official
`github.com/modelcontextprotocol/go-sdk v1.7.0`.

All 54 existing tools, scopes, CLI, `stdio` and HTTP (`/mcp`) modes, and
per-request authentication remain supported. This PR is limited to the SDK
equivalence migration; the stateless 2026 HTTP transport is deferred to a
follow-up PR.

## Verification

- Tool definitions were compared with `main` and matched for all 54 tools.
- `make lint`, `make fmt`, `go test -count=1 -race ./...`, `make build`, and
  `make tidy` pass.

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/223
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

operation/milestone/milestone_test.go

@@ -12,7 +12,7 @@ import (
 
 	"gitea.com/gitea/gitea-mcp/pkg/flag"
 
-	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
 func Test_milestoneWriteFn_dueOn(t *testing.T) {
@@ -56,7 +56,7 @@ func Test_milestoneWriteFn_dueOn(t *testing.T) {
 
 	cases := []struct {
 		name   string
-		fn     func(context.Context, mcp.CallToolRequest) (*mcp.CallToolResult, error)
+		fn     func(context.Context, map[string]any) (*mcp.CallToolResult, error)
 		method string
 		extra  map[string]any
 	}{
@@ -69,7 +69,7 @@ func Test_milestoneWriteFn_dueOn(t *testing.T) {
 			a := map[string]any{}
 			maps.Copy(a, args)
 			maps.Copy(a, tc.extra)
-			res, err := tc.fn(context.Background(), mcp.CallToolRequest{Params: mcp.CallToolParams{Arguments: a}})
+			res, err := tc.fn(context.Background(), a)
 			if err != nil || res.IsError {
 				t.Fatalf("%s err=%v result=%v", tc.name, err, res)
 			}