Align oEmbed preview handling with upstream

[cbravobernal]

Aligns the oEmbed AJAX preview path with the upstream ACF 6.8.3 approach while keeping SCF-specific copy and tests.

• Preserves valid front-end preview requests while restricting oEmbed discovery for visitors and users without content-authoring capability.
• Adds the upstream oEmbed field-type guard before preview generation.
• Updates the existing oEmbed AJAX tests to verify discovery behavior for anonymous, subscriber, administrator, and filter-overridden requests.
• Updates the 6.8.6 docs/readme wording to match the refined behavior.

Closes N/A

Use of AI Tools

This pull request was authored with assistance from OpenAI Codex. The generated changes were reviewed, tested, and adjusted before submission.

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