fix: nil TimeRange panic in MetricsViewAggregation.Export

[nishantmonu51]

• Export dereferenced q.TimeRange.TimeDimension guarded only by mv.ValidSpec.TimeDimension != "", so an export request with no TimeRange/TimeStart/TimeEnd panicked on any time-dimensioned metrics view. Now mirrors the nil-safe qryTimeDim pattern already used in Resolve.
• generateExportHeaders read qry.TimeRange.Start/.End without a nil check; RewriteQueryTimeRanges leaves qry.TimeRange unallocated for metrics views without a time dimension, so header-including exports panicked there. The date-range header is now skipped when TimeRange is nil.

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!