test(tslpatcher): strengthen merge-tslpatcher conflict artifact coverage

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Description

Adds regression tests for --merge-tslpatcher conflict artifact mode when no custom output directory is set, and for _normalize_merge_conflict_policy so invalid or missing policy values safely default to mod-a.

Type of Change

• ✅ Test addition or update

Package(s) Affected

• PyKotor (core library)

Changes Made

• test_merge_tslpatcher_artifact_default_output_dir_and_report: Asserts artifacts land in <tslpatchdata>/merge_conflicts/, exit code 1, no changes.ini, and conflicts.json contains expected keys, non-empty conflicts, and on-disk artifact paths.
• test_normalize_merge_conflict_policy_defaults_unknown: Locks behavior of _normalize_merge_conflict_policy for None, empty string, unknown strings, and valid policies.

Risky behavior now covered

• Default artifact output path: Prevents regressions where merge conflicts in artifact mode write to the wrong directory or skip README.txt / conflicts.json.
• Conflict report shape: Ensures conflicts.json remains parseable and includes conflict entries and artifact paths (downstream tooling and triage).
• Policy normalization: Unknown CLI/config values must not propagate as invalid policies; they must fall back to mod-a (consistent with run_merge_tslpatcher_workflow).

Test files added/updated

• Libraries/PyKotor/tests/tslpatcher/diff/test_diff_tslpatcher.py

Why these tests materially reduce regression risk

The prior artifact test only covered a custom merge_conflict_output_path. The new test covers the default branch in _write_merge_conflict_artifacts, which users rely on when they omit --merge-conflict-output. Policy normalization had no direct tests; a bad default could silently change conflict resolution across merges.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Test Results

PYTHONPATH=Libraries/PyKotor/src:Libraries/Utility/src QT_QPA_PLATFORM=offscreen python3 -m pytest \
  Libraries/PyKotor/tests/tslpatcher/diff/test_diff_tslpatcher.py::TestTSLPatcherFromDiff::test_merge_tslpatcher_artifact_default_output_dir_and_report \
  Libraries/PyKotor/tests/tslpatcher/diff/test_diff_tslpatcher.py::TestTSLPatcherFromDiff::test_normalize_merge_conflict_policy_defaults_unknown \
  --import-mode=importlib -m "not gui and not slow" --timeout=120
# 2 passed

Note: uv was not available in this automation environment; tests were run with python3 -m pytest after pip install of the local Libraries/PyKotor package and test deps.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Related Issues

Related to merge-tslpatcher / diff conflict artifact behavior (recent fix(diff): add conflict artifacts and unique ini sections).

Additional Notes

Tests are deterministic (temp dirs, synthetic DLG + fake install) and do not depend on game assets.