feat(validation): add YAML parser conformance warning

[hdamker]

What type of PR is this?

enhancement/feature
tests
documentation

What this PR does / why we need it:

Adds an API-scoped Python Validation check that parses each OpenAPI YAML
definition with js-yaml@5. Parser failures are reported as advisory P-037
warnings with the first parser line and column, while helper execution failures
remain error-level infrastructure findings.

The PR also adds rule metadata, unit tests for the Node helper and Python
check, and FAQ guidance for maintainers who see P-037 even though yamllint or
Spectral accepted the same file.

Local validation with this branch reports P-037 warnings at the known
subscription YAML locations in:

• ConnectedNetworkType
• DeviceReachabilityStatus
• DeviceRoamingStatus

Which issue(s) this PR fixes:

Fixes #352

Special notes for reviewers:

Local checks run:

• python3 -m pytest validation/tests/test_yaml_parser_conformance.py -q
• python3 -m pytest validation/tests/test_rule_metadata_integrity.py -q
• python3 -m pytest validation/tests -q

Changelog input

release-note
Adds an advisory validation warning for OpenAPI YAML files rejected by stricter YAML parsers.

Additional documentation

docs
documentation/validation/faq.md adds guidance for P-037.

[Kevsy]

LGTM