[T3412] FIX: Correctly set contract lines on W&P sponsorship contracts

[AlexandrePhilibert]

This fixes the W&P sponsorships amount being set when a new one is created.

Linked to CompassionCH/compassion-switzerland#1824

[greptile-apps]

Confidence Score: 5/5

Safe to merge: the exercised type-precedence cases produce the intended correspondence and standard sponsorship line behavior.

What we checked:

• We compared the payment-group onchange behavior for SC, SWP, and standard sponsorship contracts before and after the change.
• We observed that the updated logic rebuilds SC and SWP lines with amount 0 and standard sponsorship lines with amount 42, regardless of stale or absent default-type context.
• We noted that the closest existing SWP consistency test covers consistency repair rather than the group_id onchange precedence path.
• We attempted a full Odoo test, but it could not start because odoo-bin was unavailable.

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