Preserve expiry dates from nested cache collectors

[timkelty]

Nested element-cache collectors could discard their expiry when the parent collector did not have one yet. Preserve the nested duration so outer caches cannot outlive expirable content.

Discovered while reviewing #19508.

Supersedes #19507 after the branch was renamed.

[Copilot]

Pull request overview

This PR fixes a subtle bug in nested element-cache info collection where a nested collector’s expiry could be lost if the parent collector didn’t yet have an expiry, ensuring outer caches can’t outlive expirable content.

Changes:

• Preserve nested cache duration when unwinding nested cache-info collectors (even if the parent duration is currently null).
• Add a unit test that reproduces the nested-collector expiry-loss scenario and asserts the resulting duration is bounded as expected.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/services/Elements.php	Ensures nested collected expiry duration is retained when parent duration hasn’t been set yet.
tests/unit/services/ElementsTest.php	Adds coverage validating nested cache collection preserves expiry duration for the outer collector.

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