fix: pivot table sort state lost on refresh

[djbarnwal]

Due to an earlier regression, the pivot sort state was getting lost on refresh

• Pivot sort by time dimensions (e.g., timestamp_rill_TIME_GRAIN_HOUR) and minimized accessors (e.g., c0v2m0) were silently dropped during URL deserialization on page refresh
• Serialization now correctly maps _rill_ format sort IDs to URL-friendly time.{grain} format
• Deserialization reconstructs the full {timeDimension}_rill_{grain} format using metricsView.timeDimension, and no longer drops unrecognized sort IDs (like minimized accessors)
• Updated AD_BIDS_SORT_PIVOT_BY_TIME_DAY_ASC test mutation to use the realistic _rill_ format that TanStack Table actually generates

https://rilldata.slack.com/archives/C02T907FEUB/p1773768057523809

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!