Fix Contribution Transfer During Contact Merge

[shahrukh-compuco]

Overview

When two contacts are merged, civicrm_contribution.contact_id is currently rewritten to the surviving contact for every contribution attached to the deleted contact's participants, memberships, or pledges — regardless of who originally paid. This silently destroys the link between the contribution and its real payer whenever a third party (employer organisation, parent, sibling, friend, anonymous benefactor, etc.) paid on behalf of the contact being merged.

This PR scopes that rewrite to contributions actually paid by the contact being deleted. Third-party-paid contributions now stay attached to their original payer after the merge.

Before

In CRM_Dedupe_Merger::paymentSql(), each of the three UPDATE statements (pledge / membership / participant variants) had a WHERE clause that filtered only on the bridge entity's contact_id:

UPDATE IGNORE civicrm_contribution contribution
  INNER JOIN civicrm_participant_payment payment ON payment.contribution_id = contribution.id
  INNER JOIN civicrm_participant participant      ON participant.id = payment.participant_id
SET   contribution.contact_id = $mainContactId
WHERE participant.contact_id = $otherContactId

Because the WHERE only checked who the participant was — never who the payer was — every contribution attached to the deleted contact's participation got rewritten to the surviving contact, even when that contribution had originally been paid by a different contact entirely.

After

paymentSql() adds AND contribution.contact_id = $otherContactId to each WHERE` clause:

UPDATE IGNORE civicrm_contribution contribution
  INNER JOIN civicrm_participant_payment payment ON payment.contribution_id = contribution.id
  INNER JOIN civicrm_participant participant      ON participant.id = payment.participant_id
SET   contribution.contact_id = $mainContactId
WHERE participant.contact_id  = $otherContactId
  AND contribution.contact_id = $otherContactId

Behaviour matrix for the participant case (membership and pledge are
identical in shape):

participant.contact_id	contribution.contact_id	New WHERE matches?	Result
otherId	otherId	yes	rewritten to mainId (unchanged)
otherId	C (employer org)	no	stays with C ✅ (was incorrectly moved)
otherId	parent / sibling / friend	no	stays with original payer ✅
someone else	anything	no	not handled by paymentSql() (already correct under the bridge filter)
otherId	mainId	no	left alone (no double-application)

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