fix: Fix failure rate in public API

[RulaKhaled]

Purpose/Motivation

What is the feature? Why is this being done?
Apparently, failure_rate was removed in a recent migration. This PR fetches the correct failure rate for users by dividing the number of failed runs by the total number of test runs.

Links to relevant tickets

codecov/engineering-team#3582

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[seer-by-sentry]

🚨 Sentry detected 5 potential issues in your recent changes 🚨

Lower risk findings

• Line 56, api/public/v2/tests/test_test_results_view.py

  • The failure_rate is hardcoded to 1.0, which might not always be the case.

• Line 68, api/public/v2/tests/test_test_results_view.py

  • The failure_rate is hardcoded to 1.0, which might not always be the case.

• Line 100, api/public/v2/tests/test_test_results_view.py

  • The failure_rate is hardcoded to 1.0, which might not always be the case.

• Line 131, api/public/v2/tests/test_test_results_view.py

  • The failure_rate is hardcoded to 1.0, which might not always be the case.

• Line 24, api/public/v2/test_results/serializers.py: Function is missing a type annotation

  • Add type annotation for obj parameter in get_failure_rate method.

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[codecov-notifications]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
api/public/v2/test_results/serializers.py	87.50%	1 Missing ⚠️

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[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.43%. Comparing base (bd7909a) to head (de2a6e7).

Files with missing lines	Patch %	Lines
api/public/v2/test_results/serializers.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    codecov/codecov-api#1302      +/-   ##
==========================================
- Coverage   96.43%   96.43%   -0.01%     
==========================================
  Files         488      488              
  Lines       16975    16982       +7     
==========================================
+ Hits        16370    16376       +6     
- Misses        605      606       +1     

Flag	Coverage Δ
unit	96.43% <87.50%> (-0.01%)	⬇️

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