Decodes plugin titles in email subjects and bodies

[nimesh-xecurify]

Introduces a helper method to ensure plugin titles are HTML entity decoded when used in email plain-text contexts. This prevents titles containing HTML entities from displaying incorrectly.

Updates all relevant email classes to utilize this new method for consistent output.

Trac Ticket : https://meta.trac.wordpress.org/ticket/8231

[github-actions]

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Props nimeshatxecurify, dd32.

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[dd32]

AI-assisted review (prompted by @dd32):

The change looks good — clean fix for HTML entities showing literally in email subjects/bodies. One suggestion:

Consider using wp_specialchars_decode( $this->plugin->post_title, ENT_QUOTES ) instead of html_entity_decode( ..., ENT_QUOTES | ENT_HTML5, 'UTF-8' ). It's the WordPress wrapper that only decodes the entities WordPress itself encodes (&, <, >, ", '), and is what get_the_title() uses internally.

We considered get_the_title() directly, but it runs the the_title filter which could add extra markup or decorations from themes/plugins — not safe for plain-text email context. Calling wp_specialchars_decode() directly on post_title is more predictable here.

[dd32]

AI totally got this wrong :) We want to do a full decode here for Email, as plugin authors can put limited HTML into their plugin names in WordPress; Specifically entities, such as © or ®