refactor(URLGenerator): split long imagePath() policy into easy to read to helpers + one shared primitive

[joshtrichards]

• Resolves: #

Summary

This policy is long and monolithic; also includes duplicate logic. This splits the policy up into three policy-level self-documenting helpers along with a single shared primitive.

Small bug fix included: normalize '' to 'core' before computing cache storage key.

Notes

If this approach is deemed attractive, maybe we use it in some of the other long policy path resolving spots.

Related: #60481

TODO

• Resolve simple conflict

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Done. Merged master into the branch and resolved the conflict in lib/private/URLGenerator.php — the only conflict was a trivial blank-line coding standard change from master that overlapped with our new helper methods. All helper methods are preserved and the coding standard fix is incorporated.