feat!: support protocol 2026-07-28 over HTTP and add --bind (#227)

Adds MCP protocol `2026-07-28` over HTTP through the official Go SDK, and validates the `Origin` header on every request as the spec requires.

Tool and Gitea failures now come back as an ordinary `tools/call` result carrying `result.isError: true`, the way the SDK's own tool wrapper reports them. Malformed requests, unknown tools or methods, and server faults stay JSON-RPC errors.

Adds `-b, --bind` to narrow the listen address. The default still accepts every interface, so this is opt-in hardening. It matters because a request that omits `Authorization` falls back to the server's own token.

**Breaking: the HTTP endpoint no longer keeps a session per client.** What changes for a client:

1. `/mcp` accepts `POST` only, and answers `405` to `GET` or `DELETE`.
2. The server neither sends nor accepts `Mcp-Session-Id`, so there is no session handshake to perform.
3. There is no standalone SSE stream and no `Last-Event-ID` resumption. If a response stream breaks, send the whole request again under a new JSON-RPC id.

Clients that already speak current streamable HTTP need no changes. Anything relying on the session handshake or the standalone SSE stream should stay on the previous release.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/227
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

pkg/to/to.go

@@ -25,5 +25,7 @@ func TextResult(v any) (*mcp.CallToolResult, error) {
 
 func ErrorResult(err error) (*mcp.CallToolResult, error) {
 	log.Errorf("%s", err.Error())
-	return nil, err
+	var result mcp.CallToolResult
+	result.SetError(err)
+	return &result, nil
 }