fix(Database): return bytes written

[kurtisc]

Blocks RV-859

What

Fixes and adds a test for a bug introduced by #701. Found in #781

Why

The value returned was the index of the final byte, erroneously including the offset. It now only returns the length of what was copied.

Manually Testing

make all

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Explain changes to regression test captures when applicable.
• Write commit messages in agreement with our guidelines.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.

[github-actions]

Benchmark results for revision cf5bc6e:

Metric	Duration	TPS
Mean	1.518213659s	26.348
Worst	1.551798354s	25.777
Best	1.503472357s	26.605
Standard Deviation	±9.729577ms	±0.167

Full results

Run	Transfers	Duration	TPS
1	40	1.551798354s	25.777
2	40	1.525076305s	26.228
3	40	1.522627787s	26.270
4	40	1.52049963s	26.307
5	40	1.518572182s	26.341
6	40	1.513961544s	26.421
7	40	1.514890826s	26.405
8	40	1.520694102s	26.304
9	40	1.511418097s	26.465
10	40	1.51899623s	26.333
11	40	1.522575029s	26.271
12	40	1.521077753s	26.297
13	40	1.512866511s	26.440
14	40	1.503500768s	26.605
15	40	1.510007403s	26.490
16	40	1.521767844s	26.285
17	40	1.518145415s	26.348
18	40	1.512077494s	26.454
19	40	1.520247555s	26.312
20	40	1.503472357s	26.605

Compare the results above with those for the default branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.07%. Comparing base (e882b7d) to head (b1b688d).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #804      +/-   ##
==========================================
- Coverage   91.07%   91.07%   -0.01%     
==========================================
  Files         110      110              
  Lines       20831    20831              
  Branches    20831    20831              
==========================================
- Hits        18972    18971       -1     
- Misses       1486     1487       +1     
  Partials      373      373              

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