api/backend: ignore coverage for non-testable code

[vnghia]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.48%. Comparing base (b4aebd3) to head (ed155b4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nghe-proc-macro/src/backend.rs	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #561      +/-   ##
==========================================
- Coverage   74.72%   72.48%   -2.24%     
==========================================
  Files         224      183      -41     
  Lines        6626     4652    -1974     
  Branches     6626     4652    -1974     
==========================================
- Hits         4951     3372    -1579     
+ Misses       1207      847     -360     
+ Partials      468      433      -35     

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