models: grok-4.5-build and grok-4.5 collapse to one 'Grok 4.5' display name, showing duplicate rows #1029

Surfaced during the #1015 review on a real corpus. codeburn models / codeburn audit show two visually identical Grok 4.5 rows (e.g. 487 calls and 50 calls) because the distinct ids grok-4.5-build and grok-4.5 map to the same display name in the grok provider's modelDisplayName.

Expected: either the rows merge under one display name (if they are the same billable model — check pricing), or the display name distinguishes them (Grok 4.5 (build)), consistent with how other providers handle variant ids. Pre-existing; not introduced by #1015.