CE: Add provider routing + wildcard proxy contract tests

[ericflo]

Summary

• Add 17 contract tests for ProviderRouting (only/ignore/order/allow_fallbacks combinations)
• Add 5 tests for wildcard proxy endpoint prefix routing
• Verify backward compatibility with no-routing and empty-routing requests
• Add helper functions for endpoint-prefixed worker registration and routing submissions

Test plan

• All 180 existing + new tests pass (cargo nextest run -p modelrelay-server -p modelrelay-protocol -p modelrelay-cloud)
• New tests cover: only, ignore, order, allow_fallbacks (true/false/default), combined filters, endpoint prefix matching/rejection, empty prefixes legacy behavior