fix: replace err.Error() string match with errors.Is(err, pgx.ErrTxClosed)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/dav/shared.go

@@ -3,11 +3,13 @@ package dav
 import (
 	"context"
 	"crussell/clock"
+	"errors"
 	"fmt"
 	"log/slog"
 	"strings"
 	"time"
 
+	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
@@ -183,10 +185,10 @@ func (s *BaseService) CreateContact(addressBookID int, userID string, input Cont
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(context.Background()); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(context.Background()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	query := `
 		INSERT INTO dav_cards (addressbookid, uri, carddata, lastmodified, etag, size)
@@ -217,10 +219,10 @@ func (s *BaseService) UpdateContact(addressBookID int, uri string, input Contact
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(context.Background()); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(context.Background()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	query := `
 		UPDATE dav_cards
@@ -242,10 +244,10 @@ func (s *BaseService) DeleteContact(addressBookID int, uri string) error {
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(context.Background()); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(context.Background()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	query := `DELETE FROM dav_cards WHERE addressbookid = $1 AND uri = $2`
 	_, err = tx.Exec(context.Background(), query, addressBookID, uri)
@@ -267,10 +269,10 @@ func (s *BaseService) CreateEvent(calendarID int, input EventInput) error {
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(context.Background()); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(context.Background()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	query := `
 		INSERT INTO dav_calendarobjects
@@ -306,10 +308,10 @@ func (s *BaseService) UpdateEvent(calendarID int, uid string, input EventInput)
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(context.Background()); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(context.Background()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	query := `
 		UPDATE dav_calendarobjects
@@ -336,10 +338,10 @@ func (s *BaseService) DeleteEvent(calendarID int, uid string) error {
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(context.Background()); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(context.Background()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	query := `DELETE FROM dav_calendarobjects WHERE calendarid = $1 AND uid = $2`
 	_, err = tx.Exec(context.Background(), query, calendarID, uid)