ListBackups: an ExclusiveStartBackupArn with U+0000 answers "BackupArn is not valid", where DynamoDB names the account, the table or the backup id #907

What ExtendDB does

list_backups_cursor in crates/engine/src/backup.rs checks the shape with check_backup_arn before it compares the account. A shape that check refuses gets Invalid Request: BackupArn is not valid.

Measured at 9234b54 of #830 on 2026-11-07, SQLite and PostgreSQL. The account is the caller's unless the row names another:

ExclusiveStartBackupArn	ExtendDB
account segment 1234<U+0000>	ValidationException: Invalid Request: BackupArn is not valid
U+0000 in the table segment	Invalid Request: BackupArn is not valid
U+0000 in the backup id, backup/0170<U+0000>	Invalid Request: BackupArn is not valid

What DynamoDB does

Measured in us-east-1 on 2026-11-06, with SDK parameter validation off:

ExclusiveStartBackupArn	DynamoDB
account segment 1234<U+0000>	ValidationException: Invalid Request: The supplied ExclusiveStartBackupArn does not match the account
U+0000 in the table segment	Invalid Backup ARN
U+0000 in the backup id	Invalid Request: The supplied ExclusiveStartBackupArn is not a valid backup arn

An account segment of 13 digits gets the account sentence from both.

Suggested fix

Compare the account segment before the rest of the shape, and give the table segment and the backup id the two sentences above. Which other characters get each sentence was not measured.

Related: #617, #830.