Refactor: Use Internal Loader Methods for Role Metadata in MetadataRepository and tests

[srinjoydutta03]

This PR refactors MetadataRepository to replace direct calls to

• self._storage_backend.get(Snapshot.type), self._storage_backend.get(Targets.type), and self._storage_backend.get(Root.type)
• with the internal methods _storage_load_snapshot(), _storage_load_targets(), and _storage_load_root().

Unit tests are updated accordingly in test_repository.py

Tests

Local tests are sucessfully run without any errors

Closes: #757

CC: @kairoaraujo

[srinjoydutta03]

@kairoaraujo Sorry for the unnecessary commits :( , got messed up between the commit histories

[codecov]

Codecov Report

Attention: Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.

Project coverage is 75.84%. Comparing base (714a29d) to head (3319bea).
Report is 201 commits behind head on main.

Files with missing lines	Patch %	Lines
repository_service_tuf_worker/repository.py	56.52%	10 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (714a29d) and HEAD (3319bea). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (714a29d)	HEAD (3319bea)
	2	1

Additional details and impacted files

@@             Coverage Diff              @@
##              main     #758       +/-   ##
============================================
- Coverage   100.00%   75.84%   -24.16%     
============================================
  Files           15       14        -1     
  Lines         1071     1511      +440     
============================================
+ Hits          1071     1146       +75     
- Misses           0      365      +365     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.