Fix(container): fix classname resolution when multiple 's' at end

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !39184

Do not rely on getSingular($container['name'])

The current approach does not correctly convert names ending with double 's'.
For example:

• 'temps passé' → 'tempspass' → <object>tempspas (in DB)

'tempspass' when using getSingular (KO)
'tempspas' when using internal process (OK)

This PR replaces getSingular with an internal process to correctly convert the container name to the PHP class name, using:

1. Toolbox->getSystemNameFromLabel
2. PluginFieldsContainer::getClassname

'tempspass' => 'PluginFields<object>tempspas'

Screenshots (if appropriate):

[stonebuzz]

Relying on the label introduces additional issues.

For example, the getSingular function already handles a specific case for plurals ending with ss, which should remain unchanged when the function is called. To address this, I added a check that detects the presence of a double s and removes the trailing s when necessary.

[trasher]

Isn't the real issue temps passé should become tempspasse instead of tempass?

[stonebuzz]

Accented characters are systematically removed by the fields plugin (this has been the case for a long time).

I am somewhat hesitant to modify this behavior, especially considering the migration effort that would be required to restore everything correctly.