fix(oauth): Prevent validation errors from orphaned client tokens

[vpomerleau]

Because

• Sentry showed ValidationError: "[0].name" is required
• OAuth token queries use LEFT OUTER JOIN with clients table
• When a client is deleted but tokens remain (orphaned), the JOIN returns NULL
• This may be converted to undefined, which fails Joi validation

This pull request

• Add nullish coalescing in factories.ts when merging OAuth client names
• Fix shared reference bug in getDefaultClientFields() to return copy of defaults
• Add regression test for undefined client_name handling

Issue that this pull request solves

Closes: FXA-13132

Checklist

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• My commit is GPG signed.
• 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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[Copilot]

Pull request overview

This PR addresses Joi validation failures in connected-services responses when OAuth client tokens become orphaned and client_name comes back as undefined from a left join, by normalizing the display name and adding a regression test.

Changes:

• Coerce oauthClient.client_name to null (instead of propagating undefined) when populating AttachedClient.name.
• Return a copy from getDefaultClientFields() to avoid sharing the defaults object.
• Add a regression test covering undefined client_name handling.

Reviewed changes

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

File	Description
packages/fxa-shared/connected-services/factories.ts	Normalizes missing OAuth client names to null; returns cloned defaults for client field initialization.
packages/fxa-shared/test/connected-services/factories.ts	Adds regression test ensuring undefined client_name is coerced to null.

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