fix(backend): require html or text in emails.create at the type level

[cbnsndwch]

Summary

Stacked on #9010. Addresses CodeRabbit review feedback on that PR.

• Enforce the "at least one of html / text" content requirement at the type level. CreateEmailParams now intersects a union, so omitting both bodies is a compile-time error instead of a server-side one. (CodeRabbit: Major)
• Add a text-only test for emails.create to match the existing html request/response coverage. (CodeRabbit: Minor)

No changelog impact: the user-facing entry for emails.create already ships in #9010, so this carries an empty changeset.

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Latest commit: 3a84533

The changes in this PR will be included in the next version bump.

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