fix(go): emit the oneOf no-match error once instead of once per member (#24683)

The no-match branch of the generated UnmarshalJSON was wrapped in a
{{#oneOf}} section, so the error return introduced by #24349 was rendered
once per oneOf member. Only the first copy is reachable; the rest is dead
code that grows with the number of members.

Render the block once, outside the section. err is declared unconditionally
at the top of UnmarshalJSON, so the {{^oneOf}} fallback is no longer needed.
Behaviour is unchanged: the first copy is what executed before.

Regenerate the affected go samples.

fix #24661

Author: kdelay

modules/openapi-generator/src/main/resources/go/model_oneof.mustache

@@ -80,16 +80,11 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	} else if match == 1 {
 		return nil // exactly one match
 	} else { // no match
-        {{#oneOf}}
-        if err != nil {
-		   return fmt.Errorf("data failed to match schemas in oneOf({{classname}}): %v", err)
-        } else {
-           return fmt.Errorf("data failed to match schemas in oneOf({{classname}})")
-        }
-        {{/oneOf}}
-        {{^oneOf}}
-        return fmt.Errorf("data failed to match schemas in oneOf({{classname}})")
-        {{/oneOf}}
+		if err != nil {
+			return fmt.Errorf("data failed to match schemas in oneOf({{classname}}): %v", err)
+		}
+
+		return fmt.Errorf("data failed to match schemas in oneOf({{classname}})")
 	}
 	{{/discriminator}}
 	{{/useOneOfDiscriminatorLookup}}
@@ -124,16 +119,11 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	} else if match == 1 {
 		return nil // exactly one match
 	} else { // no match
-        {{#oneOf}}
-        if err != nil {
-            return fmt.Errorf("data failed to match schemas in oneOf({{classname}}): %v", err)
-        } else {
-            return fmt.Errorf("data failed to match schemas in oneOf({{classname}})")
-        }
-        {{/oneOf}}
-        {{^oneOf}}
-        return fmt.Errorf("data failed to match schemas in oneOf({{classname}})")
-        {{/oneOf}}
+		if err != nil {
+			return fmt.Errorf("data failed to match schemas in oneOf({{classname}}): %v", err)
+		}
+
+		return fmt.Errorf("data failed to match schemas in oneOf({{classname}})")
 	}
 	{{/useOneOfDiscriminatorLookup}}
 }