fix: setValidState embed API fails with tr=inf and no grain

[AdityaHegde]

Calling setValidState with tr=inf in the url param and no grain would return an error, Selected time grain: "undefined" is not valid. This is because we do not have a valid derived grain for inf time range, but an error here doesnt make sense.

Updating to only throw when the derived grain is valid. Absense of grain will be backfilled in other parts of the code. A unification of time controls calc is pending, this should be fixed there and the backfilled grain should be derived upfront.

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!

[Copilot]

Pull request overview

Allows all-time URL ranges without an explicit grain to pass validation.

Changes:

• Only reports invalid derived grains when a grain exists.
• Leaves missing grains for downstream backfilling.

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[pjain1]

There is a comment from copilot.