fix: recover from corrupted network-config.json cache

[pujitha24]

Proposed Commit Message

fix: recover from corrupted network-config.json cache

Init._write_network_config_json() read the cached
network-config.json instance file via util.load_json() with no
exception handling. A corrupted, empty, or truncated cache file
(e.g. from an interrupted write) makes json.loads() raise
JSONDecodeError, or makes util.load_json()'s root-type check raise
TypeError when the parsed value isn't a dict. Either exception was
unhandled and propagated out of _write_network_config_json(), which
is called from apply_network_config() during boot, so a bad cache
file broke network configuration entirely instead of being safely
regenerated.

Wrap the load in a try/except covering JSONDecodeError, TypeError,
and ValueError, matching the existing pattern used for the same
util.load_json() call a few lines below in this file. On failure,
log a warning and treat the cache as absent so the current
network config is written out fresh, mirroring the no-cache-file
code path just below.

Refs GH-7069

Additional Context

The root cause and fix were suggested in the issue itself. This PR
additionally catches TypeError (not just JSONDecodeError/
ValueError), since util.load_json() raises TypeError when the
cached file contains syntactically valid JSON that isn't a dict
(e.g. a truncated write leaving null or a list) -- the same
exception class already handled for an equivalent util.load_json()
call a few lines below in cloudinit/stages.py (system_info/features
cache) and in cloudinit/sources/DataSourceVMware.py.

Test Steps

Ran the affected test module directly:

python -m pytest tests/unittests/test_stages.py -q

60 passed.

The new test
test_write_network_config_json_recovers_from_corrupted_cache is
parametrized over three corrupted-cache shapes (malformed JSON, an
empty file, and valid JSON of the wrong type) and asserts that
_write_network_config_json logs a warning and rewrites the cache
instead of raising. I confirmed this test fails with an unhandled
json.decoder.JSONDecodeError against the pre-fix code and passes
after the fix.

Also ran black --check, isort --check-only, ruff check,
pylint --disable=all --enable=E, and mypy against the two
changed files (cloudinit/stages.py,
tests/unittests/test_stages.py); all clean.

This change was validated with local unit tests and static checks
only (no live/dev-stack boot was performed), since the defect is a
straightforward unhandled-exception bug reproducible with a targeted
unit test.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

Fixes #7069