[Issue] Ensure string is returned when applying filter to email template

[m2-assistant]

This issue is automatically created based on existing pull request: #32298: Ensure string is returned when applying filter to email template

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Description (*)

Added string casting within the email template filter method to ensure the return value is a string.
Currently if an exception is caught while not in developer mode a Phrase object is returned. This in turn triggers a fatal Uncaught TypeError in magento/framework/Filter/DirectiveProcessor/TemplateDirective.php:68.

Related Pull Requests

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Fixed Issues (if relevant)

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Manual testing scenarios (*)

1. Install module
   template_error.zip
2. Switch to Production mode.
3. Go to Marketing -> Newsletter Templates and create a new one.
4. Insert {{template config_path="design/email/header_template"}} into Template Content filed.
5. Preview Template.
6. We're sorry, an error has occurred while generating this content. message displayed.

Questions or comments

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Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)