refactor!: replace mcp-go with the official MCP Go SDK

- Swap the mark3labs MCP dependency for modelcontextprotocol/go-sdk v1.7.0
- Add a declarative tool definition and JSON Schema builder to the tool package
- Adapt registered tools to the official low-level handler, recovering panics and mapping failures to JSON-RPC errors
- Narrow tool handlers to take a plain argument map instead of an SDK request type
- Rewire stdio and HTTP transports onto the official server, moving Authorization parsing into receiving middleware
- Add a golden contract test that locks the exposed tool schemas, plus SDK integration and helper tests
- Add a test target and run it in the pull request workflow

BREAKING CHANGE: The exported helpers change signature. Tool.RegisterRead and
Tool.RegisterWrite now take tool.ServerTool instead of server.ServerTool, and the
annotation constructors return *mcp.ToolAnnotations from the official SDK. Callers
must build tool definitions with tool.NewDefinition and handlers with the
func(context.Context, map[string]any) signature.

The 30-second SSE heartbeat is removed because the official SDK has no equivalent.
ServerOptions.KeepAlive is deliberately not used as a substitute, since it sends MCP
ping requests and ping is removed in protocol 2026. HTTP stays stateless=false, so
new clients negotiate at most 2025-11-25.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.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)
 			}