feat: Allow no-free clusters without replication_specs in clu2adv command

[lantoli]

Description

Allow no-free clusters without replication_specs in clu2adv command

Link to any related issue(s): CLOUDP-344410

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

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[Copilot]

Pull Request Overview

This PR modifies the clu2adv command to allow conversion of non-free clusters that lack replication_specs, treating them similarly to free tier clusters by automatically generating default replication specs during conversion.

• Updated cluster identification logic to use provider_name instead of replication_specs presence
• Added default replication spec generation for clusters without explicit replication specs
• Removed the error that previously blocked conversion of clusters missing replication_specs

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

File	Description
internal/convert/const.go	Added new constants for default node count and tenant provider identification
internal/convert/clu2adv.go	Modified cluster identification logic and added default replication spec creation
internal/convert/testdata/adv2v2/errors.json	Removed error message for missing replication_specs
internal/convert/testdata/clu2adv/*.tf	Updated test data files to reflect new conversion behavior

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