fix(billing): Render unlimited reserved as ∞ in usage history

[dashed]

Billing admins reviewing past periods in the usage history table see >100% in the "Used (%)" column for any category that had unlimited reserved quota — exactly the row in the UI designed to surface overages.

The bug

usageHistory.tsx renders the per-category "Used (%)" cell with a nested ternary that special-cases RESERVED_BUDGET_QUOTA (-2) but not UNLIMITED_RESERVED (-1):

<td>
  {metricHistory.reserved === RESERVED_BUDGET_QUOTA
    ? 'N/A'
    : usagePercentage(
        convertUsageToReservedUnit(metricHistory.usage, metricHistory.category),
        metricHistory.prepaid
      )}
</td>

Inside usagePercentage:

function usagePercentage(usage: number, prepaid: number | null): string {
  if (prepaid === null || prepaid === 0) { return t('0%'); }
  if (usage > prepaid) { return '>100%'; }   // usage > -1 is always true
  return formatPercentage(usage / prepaid, 0);
}

For prepaid = UNLIMITED_RESERVED = -1, usage > -1 is always true for any non-negative usage, so the function returns '>100%'. Opposite of the truth.

The fix

Both layers, defense in depth:

1. Helper (usagePercentage) — early-return the UNLIMITED (∞) constant for unlimited prepaid, and export the helper so the unit-test layer can pin the contract:

-function usagePercentage(usage: number, prepaid: number | null): string {
+export function usagePercentage(usage: number, prepaid: number | null): string {
+  if (isUnlimitedReserved(prepaid)) {
+    return UNLIMITED;
+  }
   if (prepaid === null || prepaid === 0) {
     return t('0%');
   }

2. Call site — add a parallel isUnlimitedReserved(reserved) → UNLIMITED branch next to the existing RESERVED_BUDGET_QUOTA → 'N/A' guard:

 <td>
   {metricHistory.reserved === RESERVED_BUDGET_QUOTA
     ? 'N/A'
+    : isUnlimitedReserved(metricHistory.reserved)
+      ? UNLIMITED
     : usagePercentage(...)}
 </td>

Why both layers: the call-site guard reads the canonical metricHistory.reserved field and mirrors the neighboring RESERVED_BUDGET_QUOTA pattern. The helper guard protects against any caller — now or future — passing prepaid = -1, and enables pure unit tests of the helper contract.

Who is affected

• Enterprise orgs promoted to unlimited via correct_enterprise_reserved() (getsentry/billing/plans/__init__.py:103) — the same cohort as fix(billing): Account for gifted quantities in productIsEnabled check #113142 and fix(billing): Treat UNLIMITED_RESERVED as active quota in profiling #113254.
• Historical am3_t / am3_t_ent trial periods. create_metric_histories_from_updates (getsentry/models/billingmetrichistory.py:189) preserves the prepaid = UNLIMITED_RESERVED state on monthly rotation, so these rows persist in the usage history table indefinitely.

Tests

• describe('usagePercentage', ...) — 7 new pure-function unit tests of the exported helper (null, 0, unlimited, overage, under-quota, zero usage, exact match).
• Inside the existing component describe — two new render-level cases pin the regression: the fully-unlimited row (reserved = prepaid = -1) and the defense-in-depth divergent case (reserved > 0, prepaid = -1). The existing "overage is shown as >100%" test remains green, confirming non-unlimited rows still surface overages correctly.

Relationship to #113142 / #113254

Third and final PR in a set addressing the UNLIMITED_RESERVED = -1 sentinel:

PR	Surface
#113142	productIsEnabled + usage-overview URL selector
#113254	checkBudgetUsageFor + profiling billing banner
this PR	usagePercentage + usage history table

All three now follow the same "treat -1 as active/unlimited, not as a small negative number" convention.

Refs #113142
Refs #113254

[brendanhsentry]

It makes more sense to make this 'N/A' imo. Infinite percent used could be misleading

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀