[JENKINS-73791] Prevent PR 404 on details to break Branches scan

[Dohbedoh]

Description

JENKINS-73791: The retrieval of branches can fail the entire Branch Scanning if a BranchDiscoveryTrait strategy that involves checking on PRs is used while some PR parts cannot be retrieved with a 404 FileNotFoundException.

This is an extension of JENKINS-56996 / https://github.com/jenkinsci/github-branch-source-plugin/pull/224.

Submitter checklist

• [x] Link to JIRA ticket in description, if appropriate.
• [x] Change is code complete and matches issue description
• [x] Automated tests have been added to exercise the changes
• [ ] Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• [ ] Run the changes and verify that the change matches the issue description
• [ ] Reviewed the code
• [ ] Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• [ ] Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify

@jenkinsci/github-branch-source-plugin-developers

[Dohbedoh]

First commit to reproduce the failure. Second commit coming soon to demonstrate the fix.

[Dohbedoh]

@dwnusbaum @jeromepochat @rsandell Can we merge this ?

[BorisYaoA]

LGTM

[jglick]

Does #840 not cover this case?

[Dohbedoh]

@jglick Nope. That https://github.com/jenkinsci/github-branch-source-plugin/pull/840 is for the Organization Scan. This here is for Branch Indexing.

[Dohbedoh]

NOTE: Copilot is also an example that interrupts branch indexing. It will be handled specifically at https://github.com/jenkinsci/github-branch-source-plugin/pull/864.