backport: size-aware CRDB bulk export partitioning

[ostafen]

Balance partitions by total range bytes from SHOW RANGES ... WITH DETAILS instead of by range count, so workers process roughly equal amounts of data when range sizes are uneven. Uses a simple greedy: walk ranges in order, accumulating size, and split at the next boundary once a target of totalSize/K is reached. Skewed inputs may yield fewer than K partitions, but each one stays close to its fair share.

Falls back to range-count balancing (logged) when CRDB reports zero range_size for every range, e.g. on tables too fresh to have been sized.

Description

Testing

References

[codecov]

Codecov Report

❌ Patch coverage is 92.72727% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.41%. Comparing base (b371613) to head (7248b55).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/datastore/crdb/partitioner.go	92.73%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3092      +/-   ##
==========================================
- Coverage   75.66%   75.41%   -0.24%     
==========================================
  Files         486      502      +16     
  Lines       59433    61312    +1879     
==========================================
+ Hits        44964    46235    +1271     
- Misses      11196    11728     +532     
- Partials     3273     3349      +76     

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