chore: Move CloseTokenSource call out of defer

[lantoli]

Description

Move CloseTokenSource call out of defer.

This is an improvement so we can call Fatal (as it was before) if tf6server.Serve returns an error. We can't use defer because os.Exit in Fatal doesn't call deferred methods.

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

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• I have signed the MongoDB CLA
• I have read the contributing guides
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• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
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Further comments

[Copilot]

Pull Request Overview

This PR improves error handling in the provider's main function by moving the CloseTokenSource cleanup call out of a deferred execution. The change enables proper error handling with log.Fatal, which would otherwise skip deferred functions due to its use of os.Exit.

• Moved config.CloseTokenSource() from defer to direct call after tf6server.Serve
• Changed error handling from log.Println to log.Fatal for critical server startup failures

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[manupedrozo]

Nice