fix: increase coverage for ascend and descend scan

[SYaoJun]

Test ScanAsc and ScanDesc:

1. If the startKey is in the boundary, the function should return "kNotFound".
2. If the callback returns false, the function should return "kOK".
3. If the startKey is found in the B-tree, the function should return the expected count and value.

[codecov]

Codecov Report

Attention: Patch coverage is 89.13043% with 5 lines in your changes missing coverage. Please review.

Project coverage is 62.75%. Comparing base (3d3b882) to head (29e4793).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/btree/BasicKV.cpp	50.00%	0 Missing and 3 partials ⚠️
tests/BasicKVTest.cpp	95.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   62.36%   62.75%   +0.38%     
==========================================
  Files         119      119              
  Lines        8906     8948      +42     
  Branches     3478     3505      +27     
==========================================
+ Hits         5554     5615      +61     
+ Misses       2971     2929      -42     
- Partials      381      404      +23     

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[zz-jason]

LGTM