MongoDB Atlas shared/free tiers return a legacy dbStats shape that omits
`totalSize` (while still returning storageSize/indexSize). diagnosticService
passed that undefined straight through, so the Diagnostics UI crashed with
"Expected a finite number, got undefined".
Derive totalSize (storageSize + indexSize) at the DB and per-collection level
so it always satisfies the non-optional `totalSize: number` type. Confirmed it
is not a version removal (stock mongo:8 returns totalSize) nor a permissions
issue (missing even with atlasAdmin). Adds unit tests for the derivation.
Alex Holliday
Rabson