allow to send html emails on mailgun

[firefart]

Description

This changes allows to send HTML emails with the mailgun provider. Currently all emails are hardcoded to plain text emails.

Motivation and Context

I want to send HTML emails using the mailgun provider

How Has This Been Tested?

locally

Screenshots / Output (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Copilot]

Pull request overview

Adds support for sending HTML email bodies when using the Mailgun notification provider, instead of always sending plain text.

Changes:

• Introduce an internal Mailgun “mode” (text vs HTML) with a default of plain text.
• Add a WithHTML() option to configure the Mailgun service to send HTML bodies.
• Update Send() to build either a plain-text message or an HTML message based on the configured mode.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
service/mailgun/options.go	Adds WithHTML() option to configure HTML mode for outgoing messages.
service/mailgun/mailgun.go	Adds mode state and updates Send() to construct a text or HTML Mailgun message accordingly.

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