fix: vol 5427 certificate of roadworthiness vrm bug

[waheeduk]

Description

Lowercase vehicle registration numbers were being rejected on the certificate of roadworthiness VRN step despite a filter (Dvsa\Olcs\Transfer\Filter\Vrm) already existing to uppercase input before validation.
The filter was never being attached because the validator rule string returned by the backend includes a leading backslash (\Dvsa\Olcs\Transfer\Validators\Vrm), while ::class resolves without one, causing the equality check in ValidatorsAdder::add() to always fail silently.
Fixed by trimming the leading backslash before comparison.

Related issue: VOL-5427

Before submitting (or marking as "ready for review")

• Does the pull request title follow the conventional commit specification?
• Have you performed a self-review of the code
• Have you have added tests that prove the fix or feature is effective and working
• Did you make sure to update any documentation relating to this change?