fix: restore Magic Transit active tunnel metric

[michielappelman]

Summary

• restore the active dimension to MagicTransitMetricsQuery
• reintroduce cloudflare_magic_transit_active_tunnels
• add regression coverage for active/inactive health-check aggregation
• restore the metric documentation

Verification

The active dimension was removed in #49 because including it caused magicTransitTunnelHealthChecksAdaptiveGroups to return zero rows. A live GraphQL comparison on 2026-08-26 now succeeds both with and without the dimension:

Query	Rows	GraphQL errors
Without active	10,000	0
With active	10,000	0

The response with active contained both 0 and 1 values, confirming the field is populated again. This keeps #49’s separate correction from resultStatus === "healthy" to resultStatus === "ok".

Testing

• npx --no-install biome check
• npx --no-install tsc --noEmit
• npx --no-install vitest run (59 tests)