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>

pkg/to/to.go

@@ -7,7 +7,7 @@ import (
 	"gitea.com/gitea/gitea-mcp/pkg/flag"
 	"gitea.com/gitea/gitea-mcp/pkg/log"
 
-	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
 func TextResult(v any) (*mcp.CallToolResult, error) {
@@ -18,7 +18,9 @@ func TextResult(v any) (*mcp.CallToolResult, error) {
 	if flag.Debug {
 		log.Debugf("Text Result: %s", string(resultBytes))
 	}
-	return mcp.NewToolResultText(string(resultBytes)), nil
+	return &mcp.CallToolResult{
+		Content: []mcp.Content{&mcp.TextContent{Text: string(resultBytes)}},
+	}, nil
 }
 
 func ErrorResult(err error) (*mcp.CallToolResult, error) {