Run the Python README quickstart against a fake transport

[kriptoburak]

Goal

Keep the Python README quickstart executable without external services.

Scope

• Recreate the README's first client request in a focused test.
• Reuse the repository's mock-client patterns.
• Assert the encoded request and decoded response.
• Never contact a remote service or read credentials.
• Cover supported Pydantic versions.

Acceptance Criteria

• The test uses deterministic local fixtures.
• The test fails when the README and SDK contract diverge.
• ./scripts/test passes for supported Python and Pydantic versions.
• Keep generated SDK files and public response types unchanged.
• Update the README only when its example needs correction.

Starting Points

• README
• API tests
• Contribution guide

This task should fit within 1 working day. It supports newcomer onboarding and OpenSSF evidence.