fix: require REST shopping transport in protocol tests

[ShuoRen-TT]

Description

The discovery and version-negotiation tests intended to require the shopping
service to use the REST transport, but they wrapped a boolean comparison in
assertIsNotNone. Both True and False are non-None, so a merchant profile
advertising another transport could pass and later be treated as a REST
endpoint.

This change uses an exact equality assertion in both paths. It also aligns the
version-negotiation docstring with the expected 422 Unprocessable Content
response.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Validation

• Regression profile advertising transport: "mcp" is rejected
• Positive profile advertising transport: "rest" passes
• .venv/bin/ruff check .
• .venv/bin/ruff format --check .
• Full conformance suite against the Python Flower Shop sample: 68 passed, 1 skipped
• git diff --check

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• No additional comments are needed for this focused assertion change
• No documentation update is required
• My changes generate no new warnings
• The corrected assertions prove the fix is effective
• New and existing tests pass locally with my changes
• No dependent downstream changes are required