refactor(joiner): use EquivalenceMode constants in dedup switches

Replaces `case "shallow":` / `case "deep":` string literals with the
existing EquivalenceModeShallow / EquivalenceModeDeep typed constants
by converting the switch expression to EquivalenceMode. Resolves a
goconst lint error triggered on CI (threshold of 6+ occurrences across
the package was crossed once the new semantic-dedup tests landed).

No behavior change — the case values compare identically.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: erraggy

joiner/oas2.go

@@ -224,10 +224,10 @@ func (j *Joiner) mergeOAS2Definitions(joined, source *parser.OAS2Document, ctx d
 			case StrategyDeduplicateEquivalent:
 				// Use semantic equivalence to determine if schemas are identical
 				mode := EquivalenceModeNone
-				switch j.config.EquivalenceMode {
-				case "shallow":
+				switch EquivalenceMode(j.config.EquivalenceMode) {
+				case EquivalenceModeShallow:
 					mode = EquivalenceModeShallow
-				case "deep":
+				case EquivalenceModeDeep:
 					mode = EquivalenceModeDeep
 				}
 

joiner/oas3.go

@@ -341,10 +341,10 @@ func (j *Joiner) mergeSchemas(target, source map[string]*parser.Schema, strategy
 			case StrategyDeduplicateEquivalent:
 				// Use semantic equivalence to determine if schemas are identical
 				mode := EquivalenceModeNone
-				switch j.config.EquivalenceMode {
-				case "shallow":
+				switch EquivalenceMode(j.config.EquivalenceMode) {
+				case EquivalenceModeShallow:
 					mode = EquivalenceModeShallow
-				case "deep":
+				case EquivalenceModeDeep:
 					mode = EquivalenceModeDeep
 				}