fix(admin): fix email bounce type display

[MagentaManifold]

Because

• email bounce types are shown as enum integers instead of strings

This pull request

• make the backend endpoint return transformed strings directly.

Issue that this pull request solves

Closes: FXA-13417

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• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
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• Risky or complex parts: you'll need to add an email bounce to db manually because there's no way to simulate it yet.

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[MagentaManifold]

This seems to be present for other account search methods, so I assume this one should have it as well. Same for the other one below.

[Copilot]

Pull request overview

Fixes the admin account search payload so email bounce types/subtypes are returned as human-readable strings (instead of numeric enum values), matching what the admin panel expects to display.

Changes:

• Transform bounceType and bounceSubType from numeric DB values to string enum names in emailBounces().
• Add @Features(AdminPanelFeature.AccountSearch) metadata to emailBounces() and linkedAccounts() helper methods.

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