feat(issue): add attachment read downloads (#218)

- Add a read-only `attachment_read` tool for issue and issue comment attachments.
- Use the existing Gitea SDK attachment metadata APIs for `list` and `get`.
- Add authenticated `/attachments/{uuid}` downloads in the MCP server for `download`, with small image inline results and streamed file downloads for larger content.
- Add `-max-inline-attachment-bytes` / `GITEA_MAX_INLINE_ATTACHMENT_BYTES` to configure inline image limits.
- Update the tool tables in all README variants.

Refs: #209

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/218
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: yp05327 <576951401@qq.com>

pkg/gitea/rest_test.go

@@ -2,6 +2,11 @@ package gitea
 
 import (
 	"context"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
 	"testing"
 
 	mcpContext "gitea.com/gitea/gitea-mcp/pkg/context"
@@ -28,3 +33,32 @@ func TestTokenFromContext(t *testing.T) {
 		}
 	})
 }
+
+func TestDoJSON_LimitsErrorResponseBody(t *testing.T) {
+	payload := strings.Repeat("x", errBodySnippetSize+100)
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = io.WriteString(w, payload)
+	}))
+	defer srv.Close()
+
+	origHost := flag.Host
+	defer func() { flag.Host = origHost }()
+	flag.Host = srv.URL
+
+	status, err := DoJSON(context.Background(), http.MethodGet, "repos/owner/repo", nil, nil, nil)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if status != http.StatusBadRequest {
+		t.Fatalf("expected status %d, got %d", http.StatusBadRequest, status)
+	}
+
+	var httpErr *HTTPError
+	if !errors.As(err, &httpErr) {
+		t.Fatalf("expected HTTPError, got %T", err)
+	}
+	if len(httpErr.Body) != errBodySnippetSize {
+		t.Fatalf("expected body length %d, got %d", errBodySnippetSize, len(httpErr.Body))
+	}
+}