chore: update pivot comparison suffix

[djbarnwal]

Updates the pivot comparison suffix so that it's inline with AI generated citation URLs.

https://rilldata.slack.com/archives/C08RDR9Q26P/p1768427502052339?thread_ts=1768424348.881859&cid=C08RDR9Q26P

One point to note is that the suffix is still not in sync with what we are using across the dashboard.

The follow prefix present in measure-filter-entry.ts is used in multiple places across the codebase

export const ComparisonDeltaPreviousSuffix = "_prev";
export const ComparisonDeltaAbsoluteSuffix = "_delta";
export const ComparisonDeltaRelativeSuffix = "_delta_perc"

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!

[ericpgreen2]

Looks good. Were you able to check if this fixes the AI citation URLs? If not, would you be able to check the URLs in Mike's conversation once this lands?