feat: Add certificate retirement API for individual users and implement related helper functions

[ttak-apphelix]

Description

Adds a per-user certificate retirement endpoint to support the retirement pipeline, and refactors the existing batch endpoint to reuse shared deletion logic while adding standalone unit tests for the certificate retirement views.

Changes:

• Added POST /api/certificates/v1/retire_certs_s3_for_user to retire certificates for a single (retired) username.
• Introduced _retire_single_cert helper and refactored the batch retirement view to use it.
• Added standalone unit tests for S3 key derivation, per-cert retirement, and both endpoints.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

Related Private repo PR :

• https://github.com/edx/edx-internal/pull/14325
• feat: add retirement certificate deletion functionality to LmsApi and update retirement pipeline tubular#68

Private Jira ticket:

• https://2u-internal.atlassian.net/browse/BOMS-488

[Copilot]

Pull request overview

Adds a per-user certificate retirement endpoint to support the retirement pipeline, and refactors the existing batch endpoint to reuse shared deletion logic while adding standalone unit tests for the certificate retirement views.

Changes:

• Added POST /api/certificates/v1/retire_certs_s3_for_user to retire certificates for a single (retired) username.
• Introduced _retire_single_cert helper and refactored the batch retirement view to use it.
• Added standalone unit tests for S3 key derivation, per-cert retirement, and both endpoints.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 4 comments.

File	Description
edx_arch_experiments/certificates/views.py	Adds per-user retirement API + shared helper; refactors batch flow to reuse helper.
edx_arch_experiments/certificates/urls.py	Registers the new per-user endpoint route.
edx_arch_experiments/certificates/tests/test_views.py	Adds unit tests with LMS-module stubbing to run without edx-platform.
edx_arch_experiments/certificates/tests/__init__.py	Initializes the certificates test package.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.