customization UI: all itemProvider accesses go through the IAICustomizationItemSource (#317989)

* customization UI: all itemProvider accesses go through the IAICustomizationItemSource

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Reset cachedPromise on rejection in fetchProviderItems so transient errors can recover

Co-authored-by: aeschli <6461412+aeschli@users.noreply.github.com>

* update

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aeschli <6461412+aeschli@users.noreply.github.com>