refactor: remove unused StopOnFirstMatch field from MatchingConfig

Remove StopOnFirstMatch field that was defined but never used in the
actual matching logic.

The current implementation always evaluates all rule types and applies
a fixed precedence (Domain > MAC > Network), making the StopOnFirstMatch
field unnecessary.

Changes:
- Remove StopOnFirstMatch from MatchingConfig structs
- Update DefaultMatchingConfig() function
- Update all test cases and references
- Simplify configuration to only include Order field

This cleanup removes dead code and simplifies the configuration API
without changing any functional behavior.

cmd/cli/dns_proxy.go

@@ -393,8 +393,7 @@ func (p *prog) upstreamFor(ctx context.Context, defaultUpstreamNum string, lc *c
 		}
 
 		matchingConfig = &rulematcher.MatchingConfig{
-			Order:            order,
-			StopOnFirstMatch: lc.Policy.Matching.StopOnFirstMatch,
+			Order: order,
 		}
 	}