Harden Python NVMe tests to fail gracefully on malformed JSON

[Copilot]

Several tests assumed valid JSON command output and directly indexed nested fields. When output was malformed or schema-shifted, failures were noisy (JSONDecodeError/KeyError/AttributeError) instead of actionable test failures.

• Centralized JSON parsing + access guards (base test class)

  • Added parse_json_output(...) in tests/nvme_test.py to convert decode/type problems into explicit self.fail(...) messages with command context.
  • Added json_get(..., required=True) to enforce required keys and report missing-field failures clearly.

• Replaced fragile direct JSON access in common helpers

  • Updated core helpers in tests/nvme_test.py (get_ctrl_id, get_nsid_list, get_max_ns, get_lba_format_size, get_id_*_field_value, namespace list checks) to use guarded parsing/access.
  • Preserved existing functional behavior while making schema errors fail fast with precise diagnostics.

• Hardened JSON shape checks in format/copy test paths

  • Updated tests/nvme_format_test.py to validate lbafs presence/type and per-entry structure before use.
  • Updated tests/nvme_copy_test.py to validate elbafs presence/type and entry shape before reading pif/iterating.

• Backward-compatible create-ns output handling

  • Kept support for both JSON and legacy text output in _get_created_nsid(...), with clearer failure text when neither format yields nsid.

json_output = self.parse_json_output(result.stdout, "nvme id-ns")
lbafs = self.json_get(json_output, "lbafs", context="nvme id-ns", required=True)
self.assertIsInstance(lbafs, list, f"ERROR : ... got {type(lbafs).__name__}")