fix: [#941] separate raw mail text from views

[goravel-coder]

Summary

• Treat mail.Content.Text as raw plain-text email body content.
• Rename the HTML template field to HtmlView and add TextView for plain-text templates.
• Render HtmlView and TextView into their matching email body parts.

Closes goravel/goravel#941

Why

mail.Content.Text previously behaved like a template path, so sending raw text such as "Hello" attempted to render a template named Hello. The content API now separates raw bodies from template views, so plain text can be sent directly while template users have explicit HTML and text view fields.

func (m *WelcomeMail) Content() *mail.Content {
	return &mail.Content{
		Html: "<h1>Hello Goravel</h1>",
		Text: "Hello Goravel",
	}
}

Applications that render templates should use HtmlView and TextView, which makes multipart email behavior explicit and avoids overloading Text with two meanings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.46%. Comparing base (af5e263) to head (97ed1ae).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1491      +/-   ##
==========================================
- Coverage   69.46%   69.46%   -0.01%     
==========================================
  Files         378      378              
  Lines       29710    29713       +3     
==========================================
+ Hits        20639    20640       +1     
- Misses       8118     8119       +1     
- Partials      953      954       +1     

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