test(agentcompat): add protocol clients

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

integration/agentcompat/internal/client/transfer_rejection.go

@@ -0,0 +1,46 @@
+package client
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+type OversizeUploadProbe struct {
+	Body          io.Reader
+	ContentLength int64
+}
+
+func (client *Client) ProbeOversizeUpload(ctx context.Context, transfer TransferURL, probe OversizeUploadProbe) error {
+	validated, err := client.validateTransferURL(transfer, http.MethodPost)
+	if err != nil {
+		return err
+	}
+	if probe.Body == nil || probe.ContentLength != client.maxTransferBytes+1 {
+		return fmt.Errorf("oversize upload probe: %w", ErrInvalidConfig)
+	}
+	requestContext, cancel := client.transferContext(ctx)
+	defer cancel()
+	request, err := http.NewRequestWithContext(requestContext, http.MethodPost, validated.URL, io.LimitReader(probe.Body, probe.ContentLength))
+	if err != nil {
+		return errorsNewRedacted("create oversize transfer probe", err)
+	}
+	request.ContentLength = probe.ContentLength
+	response, err := client.transferHTTPClient().Do(request)
+	if err != nil {
+		if requestContext.Err() != nil {
+			return fmt.Errorf("oversize transfer probe: %w", requestContext.Err())
+		}
+		return errorsNewRedacted("oversize transfer probe", err)
+	}
+	defer response.Body.Close()
+	body, err := readBounded(response.Body, client.maxResponseBytes)
+	if err != nil {
+		return err
+	}
+	if response.StatusCode >= 200 && response.StatusCode < 300 {
+		return fmt.Errorf("oversize transfer probe unexpectedly succeeded: %w", ErrSemanticFailure)
+	}
+	return &HTTPError{StatusCode: response.StatusCode, Message: Redact(string(body))}
+}