reduce volume of chain data pruner test (#10062)

* reduce volume of chain data pruner test

* address matkt review: restore timeline comments and fix loop boundary

- Restore the detailed pruning timeline explanation in pruningWithFrequency
  and balPruningWithTwoBatches, adapted to use derived variable names
- Fix forkBlocksRemovedInBalOnlyMode: the second loop was starting at 45
  instead of 25; pruningMark = 280 - 256 = 24, so block 25 is the first
  unpruned block. Also fix stale comment "i <= 44" -> "i <= 24" and
  extend the upper bound to 280 (the full canonical chain length)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Sally MacFarlane <macfarla.github@gmail.com>

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Signed-off-by: Sally MacFarlane <macfarla.github@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: macfarla