Assert schema URLs for all emitted spans

Is your feature request related to a problem? Please describe.

Schema URL is part of the span contract, but many instrumentation tests do not assert it explicitly. Missing or incorrect schema URL wiring can be masked by shared helper tests or scope aggregation.

Describe the solution you'd like

Add schema URL validation for all emitted spans, using shared span assertion infrastructure where practical and local assertions for specialized cases. This will touch many tests, but the contract is important enough to verify everywhere.

Describe alternatives you've considered

N/A

Additional context

N/A

Tip

_{React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more here.}