fix(engine): preserve explicit provider overrides

[Patel230]

Summary

• preserve an explicit provider override even when a different provider has configured credentials
• retain configured-deployment diagnostics without silently replacing the caller’s requested provider
• add a regression test for the mixed configured/unconfigured-provider case

Verification

• go test ./engine -run TestEffectiveSelection -count=10 -shuffle=on
• go test ./... -race -count=1 -shuffle=on
• go vet ./...
• golangci-lint run ./... --timeout=5m (0 issues)
• ecosystem and client-layering boundary guards

This fixes the consumer-side diagnostic failure found while validating Hawk against v0.2.0 and is intended for v0.2.1.