refactor(preview): align provider lookup in PreviewManager checks

[joshtrichards]

Summary

This refactor makes preview provider support and availability checks follow the same provider-loading path as preview generation instead of reading partially initialized provider state directly.

Why

getProviders() registers core and bootstrap providers and applies provider ordering. Preview generation already resolves providers through getProviders(), but isMimeSupported() and isAvailable() currently inspect $this->providers directly after partial setup.

This means support and availability checks do not use the same provider-loading and ordering path as preview generation, and isAvailable() also currently relies on isMimeSupported() side effects to register bootstrap providers before iterating providers.

This PR:

• uses PreviewManager::getProviders() as the shared provider-loading path
• switches isMimeSupported() to inspect the provider set returned by getProviders()
• switches isAvailable() to iterate providers via getProviders()
• removes the now-redundant direct provider registration calls in isMimeSupported() and isAvailable()

This does not change provider registration rules, MIME regexes, or preview generation logic.

In edge cases, this should also reduce the risk of preview capability checks disagreeing with the provider set ultimately used for generation.

It also prepares the code for follow-up refactors (e.g. further centralization of provider matching / traversal logic shared between PreviewManager and Generator).

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