fix(tests): preserve config module identity in backup tests

[frizikk]

What does this PR do?

Fixes an order-dependent test-isolation failure in the CLI suite. The pre-update-backup fixture removed hermes_cli.config and hermes_constants directly from sys.modules. Other collected tests retained imports from the old config module, while string-based patches resolved the newly imported module, so the patched sanitizer was not called.

The fixture now relies on Hermes' path/mtime-keyed config caches instead of replacing shared module objects.

Related Issue

Fixes #73144

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• Removed direct sys.modules eviction from TestRunPreUpdateBackup setup and mode configuration in tests/hermes_cli/test_backup.py.
• Kept the backup tests' HERMES_HOME and Path.home() isolation intact.
• Restored compatibility with TestSanitizeEnvLines.test_migrate_reports_normalized_line_formatting when it runs after the backup test.

How to Test

1. On unmodified current main, run:

   pytest \
     tests/hermes_cli/test_backup.py::TestRunPreUpdateBackup::test_default_creates_quick_snapshot_only \
     tests/hermes_cli/test_config.py::TestSanitizeEnvLines::test_migrate_reports_normalized_line_formatting \
     -q -o 'addopts='

   This produces 1 passed, 1 failed: the config test captures no normalization message.
2. Apply this change and rerun the same command: 2 passed.
3. Run the focused suites:

   scripts/run_tests.sh tests/hermes_cli/test_backup.py tests/hermes_cli/test_config.py -q

   Result: 350 passed.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix
• I've run pytest tests/ -q and all tests pass (the wider CLI suite has unrelated existing failures; targeted canonical suites pass)
• I've added/retained regression coverage through the exact ordering repro
• I've tested on Linux

Documentation & Housekeeping

• Documentation N/A — test-isolation-only change
• cli-config.yaml.example N/A
• CONTRIBUTING.md / AGENTS.md N/A
• Cross-platform impact N/A — Python test-process isolation only
• Tool descriptions/schemas N/A

Screenshots / Logs

Focused validation:

163 passed
350 passed
ruff check: All checks passed

[teknium1]

Merged via the round-3 stability PR #74576 (commit 57cd922, your authorship preserved). Root fix confirmed: backup+config in one process now 114 green — this was the correct diagnosis of issue #73144 where the earlier attempt (#73304) treated the symptom. Thanks @frizikk!