fix: disable export to language when selection is invalid VSCODE-455

[nirinchev]

Description

This adds the mdb.canExportToLanguage to the extension context and uses it in the when clause for the export to language commands.

Checklist

• [ ] New tests and/or benchmarks are included
• [ ] Documentation is changed or added
• [ ] I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)

Motivation and Context

• [x] Bugfix
• [ ] New feature
• [ ] Dependency update
• [ ] Misc

Open Questions

Not sure if that's the best way to go about it - I also considered adding a the mode directly and writing a more complex when clause, but decided against it to avoid the code lens and the palette commands diverging.

Also, not sure how to test that change other than manually, so will need some pointers here.

Dependents

Types of changes

• [ ] Backport Needed
• [x] Patch (non-breaking change which fixes an issue)
• [ ] Minor (non-breaking change which adds functionality)
• [ ] Major (fix or feature that would cause existing functionality to change)

[nirinchev]

We're moving away from the current export to language functionality in favor of a copilot-based replacement. Once VSCODE-573 lands, we'll have alignment between the code lens and palette commands, making this problem go away.