test: cover zero-dependency conformance mode

[yangbaechu]

Summary

Adds a small CI check for the conformance CLI's zero-dependency manifest validation path.

The conformance tool is documented as zero-dependency, and jsonschema is optional. Existing CI installs jsonschema before running the full demo, so it only exercises the enhanced schema-validation path.

This PR adds a pre-install check using python -S to verify that manifest validation still works without site-packages.

Changes

• Run conformance-test manifest before installing dependencies
• Use python -S so optional packages such as jsonschema are not available from site-packages
• Keep the existing jsonschema-enabled conformance demo unchanged

Validation

Ran locally:

python3 -S ./conformance/bin/conformance-test manifest ./conformance/examples/ai-catalog.json
./conformance/bin/run-conformance-demo
git diff --check