CSHARP-5778: Update search index helper tests

[ajcvickers]

• Ensure index definitions are valid per https://www.mongodb.com/docs/atlas/atlas-search/index-definitions/
• Use unique index name for each variation

[Copilot]

Pull Request Overview

This PR improves test maintainability for Atlas Search Index tests by refactoring index name generation and addressing test isolation issues. The changes ensure that tests with different parameters (async vs sync, with/without fields) create uniquely named indexes to avoid conflicts.

• Introduced a helper method CreateIndexName to generate unique index names based on test parameters
• Updated index definitions to use more realistic field configurations
• Refactored test methods to use the new helper and apply correct index definitions based on test parameters

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[codeowners-service-app]

Assigned jordan-smith721 for team dbx-csharp-dotnet because papafe is out of office.

[papafe]

Assigned jordan-smith721 for team dbx-csharp-dotnet because papafe is out of office.

I'm not out of office anymore by the way 😁

[papafe]

LGTM!

There is just an extra line

[ajcvickers]

@papafe Thanks! Fixed that line.

[ajcvickers]

@papafe Can you approve again, since including the Copilot commit reset the Approved status?