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>

pkg/tool/definition_test.go

@@ -0,0 +1,71 @@
+package tool
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+func TestNewDefinition(t *testing.T) {
+	annotations := &mcp.ToolAnnotations{Title: "Example", ReadOnlyHint: true}
+	definition := NewDefinition(
+		"example",
+		"Example tool",
+		annotations,
+		String("owner", Required(), Description("repository owner"), Enum("one", "two"), Default("one")),
+		Number("page", Required(), Default(1), Minimum(1)),
+		Boolean("draft"),
+		Array("labels", Items(map[string]any{"type": "string"})),
+		Object("inputs", Description("workflow inputs")),
+	)
+
+	if definition.Name != "example" || definition.Description != "Example tool" {
+		t.Fatalf("definition = %#v", definition)
+	}
+	if definition.Annotations != annotations {
+		t.Fatal("NewDefinition did not preserve annotations")
+	}
+
+	want := map[string]any{
+		"type": "object",
+		"properties": map[string]any{
+			"owner": map[string]any{
+				"type":        "string",
+				"description": "repository owner",
+				"enum":        []string{"one", "two"},
+				"default":     "one",
+			},
+			"page": map[string]any{
+				"type":    "number",
+				"default": 1,
+				"minimum": float64(1),
+			},
+			"draft": map[string]any{"type": "boolean"},
+			"labels": map[string]any{
+				"type":  "array",
+				"items": map[string]any{"type": "string"},
+			},
+			"inputs": map[string]any{
+				"type":        "object",
+				"properties":  map[string]any{},
+				"description": "workflow inputs",
+			},
+		},
+		"required": []string{"owner", "page"},
+	}
+	if !reflect.DeepEqual(definition.InputSchema, want) {
+		t.Errorf("InputSchema = %#v, want %#v", definition.InputSchema, want)
+	}
+}
+
+func TestNewDefinitionWithoutRequiredProperties(t *testing.T) {
+	definition := NewDefinition("empty", "", nil)
+	schema := definition.InputSchema.(map[string]any)
+	if _, ok := schema["required"]; ok {
+		t.Errorf("InputSchema unexpectedly contains required: %#v", schema)
+	}
+	if got := schema["properties"]; !reflect.DeepEqual(got, map[string]any{}) {
+		t.Errorf("properties = %#v, want empty map", got)
+	}
+}