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 #19505.

[Copilot]

Pull request overview

This PR fixes an edge case in craft\services\Elements cache-info collection where a nested cache collector’s computed expiry duration could be lost when unwinding back to an outer collector that hadn’t set a duration yet—allowing outer caches to potentially outlive expirable nested content.

Changes:

• Preserve a nested collector’s $duration when the parent collector’s duration is currently null.
• Add a unit test covering nested cache-info collection where only the inner collector sets an expiry date.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/services/Elements.php	Ensures nested cache duration is propagated to the parent collector when the parent has no duration yet.
tests/unit/services/ElementsTest.php	Adds regression coverage asserting nested cache expiry duration survives unwinding to the outer collector.

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