test(opennebula): validate gen_conf() output against network-config-v2 schema

[mcanevet]

Proposed Commit Message

```
test(opennebula): validate gen_conf() output against network-config-v2 schema

Add an autouse fixture to TestOpenNebulaNetwork that wraps gen_conf() and
runs jsonschema validation against schema-network-config-v2.json on every
call, so all existing tests implicitly assert schema conformance without
requiring dedicated per-field type checks.
```

Additional Context

OpenNebula's gen_conf() directly produces a Netplan v2 dict. Previously,
no test validated the output against the network-config-v2 JSON schema, so
type errors (e.g. a field being a string when the schema requires an integer)
could go undetected. The autouse fixture catches any such regression across
all existing and future tests in the class for free.

Test Steps

tox -e py3 -- tests/unittests/sources/test_opennebula.py

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>)

[mcanevet]

Unit tests fails precisely because this new test shows a bug that #6856 fixes

[Copilot]

Pull request overview

Adds schema validation to OpenNebula network unit tests to ensure OpenNebulaNetwork.gen_conf() continues to emit a Netplan v2 config that conforms to cloud-init’s schema-network-config-v2.json.

Changes:

• Import jsonschema and cloud-init schema helpers in the OpenNebula unit tests.
• Add an autouse pytest fixture on TestOpenNebulaNetwork that monkeypatches gen_conf() to validate its return value against the NETWORK_CONFIG_V2 schema on every call.

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[blackboxsw]

This is a good utility fixture. Thank you for this @mcanevet. I could see something like this being adapted to be useful for any DataSource*.network_config unittest test as well.

[blackboxsw]

I appreciate this wrapper which validates each gen_conf return value called. This definitely can help raise awareness about invalid net config responses returned by changes OpenNebula gen_conf in subsequent PRs.