Refactor a service responsible for creating a pipeline

What does this MR do?

Reactor pipeline creation service

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Has been reviewed by Backend

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#37563
Closes #34415 (closed)

restored source branch backstage/gb/refactor-pipeline-create-service

added 1 commit

• 9776dbda - Use tag/branch methods to check if pipeline ref exists

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added 2 commits

• 3420c0f7 - Fix post receive specs regarding pipeline creation
• 7cfaccd6 - Fix code style offenses in pipeline create services

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added 1 commit

• 61dc0b7d - Extract pipeline persistence to a separate chain class

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added 1 commit

• 3e60d62c - Add class that handles pipeline creation sequence

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added 2 commits

• 609fa45f - Split pipeline chain builder validation class
• fa3fb23f - Move pipeline builder validation chain to a module

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added 2 commits

• da15b388 - Add specs for pipeline builder abilities validator
• 1a8777c8 - Fix coding style offenses in pipeline chain classes

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added 2 commits

• 53cad500 - Add missing tests for pipeline chain access validator
• f6bd832f - Fix some code style offenses in pipeline chain classes

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unmarked as a Work In Progress

marked the checklist item Tests added for this feature/bug as completed

added CI/CD backstage pipeline labels

changed milestone to %10.1

added 1 commit

• 835bdcb8 - Add specs for pipeline chain builder sequence class

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mentioned in issue #34415 (closed)

changed the description

@ayufan Please review (after merge request diffs are regenerated with 057a8b70). Thanks!

assigned to @ayufan

added 1 commit

• 057a8b70 - Add test for head pipeline assignment when skipped

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added 1 commit

• 26e73c2e - Add some minor improvements to pipeline creation chain

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resolved all discussions

mentioned in issue #38460

approved this merge request

mentioned in commit 0bba522f

merged