Fix getDiskUsage and add getCollectionStats

[delthas]

Summary

• Fix getDiskUsage: Was reading the nonexistent fsFreeSize field from MongoDB's dbStats command, always returning 0 for available/free space. Now correctly computes free space as fsTotalSize - fsUsedSize.
• Add getCollectionStats: New method on MongoClientInterface that exposes per-collection stats via the collStats command. Returns the raw stats object including indexSizes (per-index size breakdown) and totalIndexSize.

Context

The lifecycle conductor (BB-753) needs to check available disk space before creating v2 indexes on MongoDB. When the MongoDB PV is nearly full, attempting index creation can fail (ENOSPC) or further pressure the disk. The conductor uses getDiskUsage for filesystem free space and getCollectionStats to get the _id_ index size as an estimate for the cost of creating lifecycle indexes.

Design decisions

• getDiskUsage keeps its existing return shape ({ available, free, total }) to avoid breaking the interface contract, just fixes the computation.
• getCollectionStats passes stats through directly from MongoDB rather than reshaping — the caller extracts what it needs.
• Uses db.command({ collStats }) instead of the deprecated collection.stats() which was removed from the MongoDB Node.js driver type definitions.
• Both calls are metadata-only (~3ms and ~8ms respectively on a 1M-document collection), no collection scans or locks.

[bert-e]

Hello delthas,

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[claude]

LGTM

Review by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.38%. Comparing base (eefeb8b) to head (a971a53).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           development/8.3    #2605      +/-   ##
===================================================
+ Coverage            73.37%   73.38%   +0.01%     
===================================================
  Files                  222      222              
  Lines                18160    18168       +8     
  Branches              3761     3761              
===================================================
+ Hits                 13324    13332       +8     
  Misses                4831     4831              
  Partials                 5        5              

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[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

• the author

• 2 peers