classic-theme-helper: Add notice that Custom Content Types are primarily for classic themes

[agent-sandbox-automattic]

What

Adds an informational notice in Settings → Writing under the "Your Custom Content Types" section heading, explaining that Portfolio and Testimonials features are primarily designed for classic themes. The notice includes a link to the Jetpack support guide at https://jetpack.com/support/custom-content-types/.

Why

Users on block themes (e.g. Videomaker) were disabling Portfolio without realizing that doing so could also remove the toggle from Settings → Writing, making it appear the feature was gone entirely. This simple static notice aligns with the stakeholder's request to add descriptive text under the section heading clarifying the classic-theme nature of these features.

How

Modified jetpack_cpt_section_callback() in projects/packages/classic-theme-helper/src/custom-content-types.php:

• The notice is always visible (no conditional on theme type) — it's a general informational message
• Uses Redirect::get_url( 'jetpack-support-custom-content-types' ) when the Redirect class is available, with a direct URL fallback
• Properly escaped with wp_kses(), esc_url(), and __() with the jetpack-classic-theme-helper text domain
• No JavaScript, no FSE-detection logic — keeps it simple as requested

Test plan

• Visit Settings → Writing on a WordPress site with Jetpack's classic-theme-helper active
• Confirm the notice "These features are primarily designed for classic themes. Learn more." appears under the "Your Custom Content Types" heading
• Confirm the "Learn more" link points to the custom content types support page
• Verify the notice appears on both classic and block themes

🤖 Generated with Claude Code

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Linear: DOTCOM-17145

This PR was auto-generated by AI Coder. Please review carefully before merging.

[github-actions]

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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/classic-theme-helper/src/custom-content-types.php	0/109 (0.00%)	0.00%	16 💔

Full summary · PHP report · JS report

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