Group small-entry transfers with div_ceil so exact-multiple sizes stop overshooting by a chunk

[CleanCut]

Both pull_small_entries and download_small_entries sized their chunk groups with the (total / segment) + 1 idiom the repo guidelines warn against. When total_size is an exact multiple of the segment size it asked for one more group than needed, yielding marginally smaller groups.

Swapping in div_ceil on its own would have introduced a panic: it returns 0 when every entry is zero bytes, and the next line divides entries.len() by that count. The .max(1) floor keeps the divisor non-zero, which the + 1 form had been providing by accident.

Fixed a similar problem with chunk_size, where it should only be 0 when the number of entries is 0.

ENG-1229

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved downloads for empty and very small files.
  • Prevented download operations from calculating zero-sized chunks, ensuring reliable transfer behavior.

Walkthrough

The small-file pull and download paths now use ceiling division and enforce a minimum chunk count of one.

Changes

Small-file chunking

Layer / File(s)	Summary
Update small-file chunk counts crates/liboxen/src/core/v_latest/fetch.rs	pull_small_entries and download_small_entries now use ceiling division with a minimum of one chunk.

Estimated code review effort: 2 (Simple) | ~5 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: using ceiling division to prevent an unnecessary chunk for exact-multiple sizes.
Description check	✅ Passed	The description directly explains the chunk-sizing changes, the zero-divisor safeguard, and the related edge cases.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cleancut/fetch-div-ceil

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/liboxen/src/core/v_latest/fetch.rs`:
- Around line 731-732: Update pull_small_entries in
crates/liboxen/src/core/v_latest/fetch.rs at lines 731-732 so its
entries-per-group calculation uses ceiling division via
entries.len().div_ceil(num_chunks). Apply the same change in
download_small_entries at lines 974-975; both sites must use ceiling division to
avoid extra requests for non-divisible entry counts.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between ebab2cc and 45d58ea.

📒 Files selected for processing (1)

• crates/liboxen/src/core/v_latest/fetch.rs