Abilities API: extend core/get-environment-info with fields and site health

[itzmekhokan]

Add optional fields input to mirror core/get-site-info. Expose site_health from the health-check-site-status-result transient only (no synchronous tests), including counts, actionable issues (recommended/critical), and truncation when more than ten issues are cached.

Persist issue summaries in the Site Health transient from the weekly cron and from the Site Health screen AJAX handler, with merge behavior when the client omits the issues payload.

Trac ticket: https://core.trac.wordpress.org/ticket/65232

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Props khokansardar, apermo.

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[itzmekhokan]

@apermo Thanks for reviewing, I've addressed the following fixes.