test: add test case for exporting custom block

[nperez0111]

Summary

This adds a test case for checking the format conversions for custom blocks, which seemed to have been missing, we already were testing inline content & react, so that is omitted here.

Rationale

Changes

Impact

Testing

Screenshots/Video

Checklist

• Code follows the project's coding standards.
• Unit tests covering the new feature have been added.
• All existing tests pass.
• The documentation has been updated to reflect the new feature

Additional Notes

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Project	Deployment	Preview	Updated (UTC)
blocknote	Ready	Preview	Nov 3, 2025 2:19pm
blocknote-website	Ready	Preview	Nov 3, 2025 2:19pm

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commit: 7d2b928