Issue Labeler - Description Textbox: Display over-limit prompt: "exceeds by N characters" when the description exceeds 100 characters.

# Description

Currently, GitHub Issue labeler shows a generic error for overlong descriptions:

`Description is too long (maximum is 100 characters)` (see screenshot below).

This increases the time users spend editing the **Description** field in cases of overlong description because they cannot see how many characters to remove without using an external character counter tool.

# Suggested Solution

* Prompt the minimum number of characters to be reduced. For instance, " Trim the message by 17 characters (limit: 100)".
or
* Prompt the number of characters over the limit. For instance, "Over by 17 characters (limit:100)".

# Alternate Solution

* Continue with current sub-optimal error prompt which requires users to use an external tool to determine minimum number of characters to trim.

# Code to demonstrate issue

# Checklist for Completion

* [ ] Add the logic to compute `excess - max(0, input_length - 100)` when input > 100
* [ ] Customize the error prompter to prompt one of the suggested solutions
* [ ] Add unit / integration tests for boundary / edge cases (99 / 100 / 101 characters)
* [ ] Update relevant documentation for the Description field error behavior

Commencez par localiser la validation du champ Description et ses tests existants de messages d’erreur dans la TypeScript GitHub Action. Vérifiez les cas limites de 99, 100 et 101 caractères, puis ajoutez la couverture nécessaire et mettez à jour la documentation pertinente du champ Description afin que l’erreur indique clairement le dépassement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.