feat(errors): pass context cancellation through asAPIError

Surface context.Canceled/DeadlineExceeded unchanged (even when wrapped
by an RPC error) so callers can errors.Is on ctx.Err() instead of seeing
a synthetic Bot API error.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ernado

errors_map.go

@@ -1,6 +1,7 @@
 package botapi
 
 import (
+	"context"
 	"errors"
 	"net/http"
 	"strconv"
@@ -120,6 +121,11 @@ func asAPIError(err error) error {
 	if errors.As(err, &already) {
 		return err
 	}
+	// Context cancellation/deadline always passes through unchanged (even if it
+	// happens to be wrapped by an RPC error) so callers can errors.Is on it.
+	if errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
+		return err
+	}
 	rpcErr, ok := tgerr.As(err)
 	if !ok {
 		return err

errors_map_test.go

@@ -3,6 +3,7 @@ package botapi
 import (
 	"context"
 	"errors"
+	"fmt"
 	"testing"
 
 	"github.com/gotd/td/tgerr"
@@ -19,6 +20,19 @@ func TestAsAPIError(t *testing.T) {
 			t.Fatalf("non-RPC error should pass through, got %v", got)
 		}
 	})
+	t.Run("PassThroughDeadline", func(t *testing.T) {
+		if got := asAPIError(context.DeadlineExceeded); !errors.Is(got, context.DeadlineExceeded) {
+			t.Fatalf("deadline error should pass through, got %v", got)
+		}
+	})
+	t.Run("ContextWrappedByRPCStillPasses", func(t *testing.T) {
+		// An RPC-typed error that wraps a cancellation must still surface the
+		// cancellation so callers can branch on ctx.Err().
+		wrapped := fmt.Errorf("rpc: %w", context.Canceled)
+		if got := asAPIError(wrapped); !errors.Is(got, context.Canceled) {
+			t.Fatalf("wrapped cancellation should pass through, got %v", got)
+		}
+	})
 	t.Run("AlreadyAPIError", func(t *testing.T) {
 		in := &Error{Code: 400, Description: "x"}
 		if got := asAPIError(in); got != error(in) {