fix(analytics): resolve account email/name in Utilization chart and fix tooltip stacking context

[ZaimMarzuki]

Summary

Fixes two UX issues on the Provider Utilization page (/dashboard/analytics/utilization) in Account Split mode:

1. Raw UUID shown in Chart lines & tooltip: Resolves account email/name for connection keys (provider:connectionId) instead of displaying the raw internal connection ID in the chart legend and hover tooltip (follow-up to fix(dashboard): show account email/name in Utilization Account Split cards #10939).
2. Tooltip text bleed-through: Fixes the chart tooltip appearing translucent or behind chart legend items when hovering.

Key Changes

• Account Display Name Resolution (ProviderUtilizationTab.tsx):
  • Defines resolveDisplayName callback that checks aggregateBy === "connection", parses provider:connectionId, and queries data?.connectionMeta to retrieve the friendly account name/email via getAccountDisplayName.
  • Passes resolveDisplayName to ProviderCharts so both SVG lines and the Recharts tooltip/legend display friendly names.
• Tooltip Stacking & Visibility Fix (ProviderCharts.tsx):
  • Moved <Tooltip /> after <Legend /> in JSX order so the SVG DOM painting layer places the tooltip above legend items.
  • Added wrapperStyle={{ zIndex: 1000, pointerEvents: "none" }} on Recharts <Tooltip /> and applied opacity: 1 with boxShadow: "var(--shadow-elevated)" on its contentStyle so text and lines underneath do not bleed through.
• Tests & Changelog:
  • Added unit test suite in tests/unit/ui/provider-utilization-chart.test.tsx testing display name resolution and tooltip zIndex / DOM order.
  • Added changelog fragment in changelog.d/fixes/utilization-account-chart-display.md.

[diegosouzapw]

Solid, focused fix — both the raw-UUID-in-chart and the tooltip-behind-legend issues are
real and this addresses them cleanly, reusing one resolveDisplayName callback instead of
duplicating the resolution logic. 2/2 new tests pass locally. Merge-ready as-is.

[diegosouzapw]

Thanks @ZaimMarzuki — merging via the release merge-train. Validated in local merge-train (.claude/worktrees/merge-train-20260918-111718-suite.log) on the devbox @ train tip 7bb373fba5e241964c0ffb17bd03a804700839bb, boarded with 55 sibling PRs: typecheck:core, file-size, complexity, cognitive-complexity, changelog-integrity green; 747/747 changed-area node:test cases + 476/476 vitest green (fast parity mode — the full suite ran today on the release tip via the base-red train and runs again on the 3b train). Merged --admin per merge-gates §7.