use current page and teaser for the description

[pfefferle]

Fixes #9

The description was always built from the whole post_content, so every page of a multipage post got the same one. It now uses core's generate_postdata() to pick the page from the page query var (works for the classic <!--nextpage--> and the block form).

While at it, the description is cut at the <!--more--> tag, the same way we do it in the ActivityPub plugin. That is a small behavior change for posts with a more tag, they get the teaser instead of the first 55 words.

og:url still points to the first page, I left that alone for now.

[Copilot]

🟢 Approval recommended

The changes are fully reviewed, tested, and have no unresolved blocking issues.

Pull request overview

Updates Open Graph descriptions to use the current multipage section and teaser content.

Changes:

• Selects the current page with generate_postdata().
• Stops descriptions at <!--more-->.
• Adds regression tests for both behaviors.

File summaries

File	Summary
tests/phpunit/tests/class-test-opengraph.php	Adds multipage and teaser coverage.
opengraph.php	Implements page-specific teaser extraction.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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