fix(auth): avoid mutating opts param in fastifyAuth

[Fdawgs]

The fastifyAuth function was mutating the opts parameter passed by the caller when setting the default relation. This could cause unexpected side effects for users who reuse the options object.

This change creates a new pluginOptions object with the resolved defaultRelation property instead of modifying the original input.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull request overview

This PR fixes a bug where the fastifyAuth function was mutating the opts parameter when setting the default relation. The fix ensures that plugin options objects can be safely reused without unexpected side effects.

Key changes:

• Replaced direct mutation of opts.defaultRelation with creation of a new pluginOptions object
• Added test coverage to verify that the original opts parameter remains unchanged

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
auth.js	Creates new pluginOptions object instead of mutating input opts parameter
test/auth.test.js	Adds test to ensure opts parameter is not mutated during plugin registration

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[is2ei]

LGTM!