chore: add core transcript unit tests

[yuroitaki]

This adds the remaining unit tests that are missing from #608. Some minor refactoring to create reusable test fixtures are included too.

[codecov]

Codecov Report

Attention: Patch coverage is 99.46667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 55.43%. Comparing base (faab999) to head (bbb6c93).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/core/src/fixtures.rs	97.64%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #649      +/-   ##
==========================================
+ Coverage   54.48%   55.43%   +0.94%     
==========================================
  Files         193      193              
  Lines       20618    20842     +224     
==========================================
+ Hits        11234    11553     +319     
+ Misses       9384     9289      -95     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[themighty1]

Nice work, pls fix the formating as per my comment and feel free to merge.