Remove test workaround from list_repo.py, add proper error handling (#2392)

[mr-raj12]

Description

Remove test-specific workaround from production code and fix the underlying test infrastructure issue.
Changes:

• src/vorta/borg/list_repo.py: Remove if not result['data']: result['data'] = {} workaround, add proper error handling with logging for unexpected data
  types
• tests/unit/conftest.py: Add mock_borg_popen fixture that provides fresh file handles for each Popen call, supporting job chaining scenarios
• tests/unit/test_archives.py: Update test_repo_prune to use new fixture (prune triggers list refresh = 2 jobs)

Related Issue

Closes #2392

Motivation and Context

Production code contained a test-specific workaround that silently masked potential errors. The root cause was test infrastructure reusing file handles across
chained jobs - when job A (prune) read from a file handle, job B (list) would get empty content because the file position was at EOF.

How Has This Been Tested?

• Verified same tests pass/fail on both master and fix branch (no regressions)
• Linting passes (ruff check)
• Full verification will happen on CI (Linux/macOS) since the affected tests use os.set_blocking() which doesn't work on Windows

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.