Add billing currency filter to Data Explorer dashboard (#2093)

[RolandKrummenacher]

Summary

Adds a Currency parameter to the FinOps hub Data Explorer dashboard so cost totals and savings KPIs can be scoped to a single billing currency. Implements Phase 1 items 1 + 2 of #2093; remaining items (Phase 1 item 3, savings-plan FX distortion, Phase 2 USD normalization) continue to be tracked there.

Why

The dashboard aggregates BilledCost / EffectiveCost / ContractedCost / ListCost across rows without any BillingCurrency awareness. For any tenant with more than one billing currency (EA→MCA transitions, M&A, CSP partners with customers in multiple currencies, multi-cloud via FOCUS), the resulting totals and savings rates are mathematically invalid — currencies get summed as if they were comparable numbers.

The CostsPlus base query feeds ~74 of the 86 dashboard queries, so adding the filter there propagates the fix everywhere downstream with minimal blast radius.

Changes

1. New selectedBillingCurrency parameter (displayed as Currency)

• Scalar, string, sourced from distinct BillingCurrency over the hub's configured retention window.
• Wired into the CostsPlus base query as | where isempty(selectedBillingCurrency) or BillingCurrency == selectedBillingCurrency, immediately after the time-range filter. Inherited by all ~74 downstream tile queries through CostsPlus and its derived base variables (CostsThisMonth, CostsLastMonth, CostsByMonth, CostsByDay, CostsByDayAHB).
• Same filter also applied to the one tile that bypassed CostsPlus — the Summary of list, contracted, and effective cost alignment data-quality table on the Data ingestion page (queries Costs() directly).

2. Conditional ⚠️ All option — implicit mixed-currency warning

Rather than adding a separate warning tile (no clean place on the dense Summary page), the warning is embedded into the parameter dropdown itself:

• Single-currency tenants: dropdown shows only their currency. No "All" option, no noise.
• Multi-currency tenants: dropdown opens with ⚠️ All as the default. The warning is visible the instant the dashboard loads, and picking any specific currency removes it automatically.

This is done via a synthetic row unioned into the parameter query only when dcount(BillingCurrency) > 1, with an empty BillingCurrency value that the filter treats as "no filter applied".

Scope / what this PR does NOT do

Phase 1 item 3 from the issue (replace sum(ListCost) - sum(EffectiveCost) patterns with sum(x_TotalSavings) etc.) is intentionally out of scope — orthogonal to currency handling and warrants independent review of whether the row-level clamped savings columns actually produce different totals.

Phase 2 (USD normalization toggle) is also deferred — blocked on x_*InUsd column coverage across ingestion paths per the issue.

Coverage audit

All 87 queries in the dashboard were reviewed:

• ✅ 86 of 87 inherit the filter through CostsPlus (directly or via the five derived base variables).
• ✅ .show cluster extents queries (ingestion row counts) — no cost aggregation, no currency risk.
• ✅ Settings/metadata queries (HubSettings, range) — no cost data.
• ✅ The parameter-population query — intentionally unfiltered so the dropdown can list all currencies.
• ✅ The one outlier that queried Costs() directly (data-quality tile on the ingestion page) — now also filtered.

Test plan

• Open the dashboard in ADX against a single-currency environment; confirm the Currency chip shows that currency only, no "All", and totals are unchanged vs. pre-PR.
• Open against a multi-currency environment; confirm the Currency chip defaults to ⚠️ All and the warning symbol is visible in the parameter bar.
• Switch the Currency filter to a specific currency in the multi-currency environment; confirm all tiles (including the cost-alignment diagnostic table on the Data ingestion page) scope correctly.
• Revert the Currency filter to "⚠️ All"; confirm totals return to the mixed (pre-PR) behavior, with the warning marker now visible in the dropdown.
• npm run build passes (already verified).

🤖 Generated with Claude Code

[RolandKrummenacher]

@copilot resolve the merge conflicts in this pull request