Close label files reliably after reads and writes

[vietanhdev]

Summary

• close JSON label files deterministically after each read and write
• prevent descriptor leaks and Windows file-lock surprises during overwrite/delete workflows
• add regression tests for context closure and repeated save → load → delete

Validation

• focused tests run with ResourceWarning promoted to an error: 5 passed
• full suite: 142 passed, 13 optional-model tests skipped
• ruff check anylabeling/views/labeling/label_file.py tests/test_label_file.py
• manual repeated overwrite/load/delete workflow passed in the dedicated .venv