Refactor Import placeholder push reference service

What does this MR do and why?

Refactor the service to log errors, handle composite key and raise error in development environment.

The service currently isn't working on production. This MR is part of a series of MRs that will integrate user contribution mapping in Direct Transfer.

Related to: #443557 (closed)

MR that implemented the service: !156704 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

The service is not used by any class, so to test it, we need to use the Rails console. Here are some examples of how to use the service:

MergeRequest reference

Import::PlaceholderReferences::PushService.from_record(import_source: Import::SOURCE_DIRECT_TRANSFER, import_uid: 1, source_user: Import::SourceUser.first, record: MergeRequest.new(id: 1), user_reference_column: :author_id).execute

IssueAssignee reference

Import::PlaceholderReferences::PushService.from_record(import_source: Import::SOURCE_DIRECT_TRANSFER, import_uid: 1, source_user: Import::SourceUser.first, record: IssueAssignee.new(user_id: 1, issue_id: 2), user_reference_column: :user_id).execute

Invalid reference

Import::PlaceholderReferences::PushService.from_record(import_source: Import::SOURCE_DIRECT_TRANSFER, import_uid: 1, source_user: Import::SourceUser.first, record: IssueAssignee.new(issue_id: 2), user_reference_column: :user_id).execute