fix: timestamp column selection in explore not persiting

[AdityaHegde]

Timestamp column selection in explore does not persist across refreshes. This is because we were checking the selection against non-time dimensions which always failed.

Also added some missing unit tests for converting url params to explore state.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!