fix(go): avoid duplicate unknown enum defaults

[SJvaca30]

Fixes #23844.

What

• Adds a model-specific prefix to the synthetic Go unknown_default_open_api enum fallback when enumClassPrefix=false.
• Leaves user-defined enum constants unprefixed.
• Adds regression coverage with two enum models generated into the same Go package.

Testing

• ./mvnw -pl modules/openapi-generator -am -Dtest=org.openapitools.codegen.go.GoClientCodegenTest#testEnumUnknownDefaultCaseUsesModelSpecificNamesWhenEnumClassPrefixDisabled -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl modules/openapi-generator -am -Dtest=org.openapitools.codegen.go.GoClientCodegenTest -Dsurefire.failIfNoSpecifiedTests=false test
• git diff --check

---

Summary by cubic

Avoids duplicate Go enum fallback constants by prefixing the synthetic unknown default with the model name when enumClassPrefix=false. Fixes #23844.

• Bug Fixes
  • Prefixes the unknown_default_open_api fallback with the model name when enumUnknownDefaultCase=true and enumClassPrefix=false, preventing collisions across models in the same package.
  • Leaves user-defined enum constants unchanged.
  • Adds regression test with two enum models to verify unique names (e.g., STATUSA_UNKNOWN_DEFAULT_OPEN_API, STATUSB_UNKNOWN_DEFAULT_OPEN_API).
  • Documents the prefixing behavior in the Go generator docs.

^{Written for commit e230699. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[wing328]

please add a docstring explaining what this function does

a few inline comments explaining the if blocks below would be helpful too

[SJvaca30]

Added Javadoc for prefixEnumUnknownDefaultCase and inline comments for the guard conditions and fallback-name update.

[wing328]

tested locally and the result is good (no compilation error)

--- a/model_status_a.go
+++ b/model_status_a.go
@@ -22,7 +22,7 @@ type StatusA string
 const (
        ACTIVE StatusA = "active"
        INACTIVE StatusA = "inactive"
-       UNKNOWN_DEFAULT_OPEN_API StatusA = "unknown_default_open_api"
+       STATUSA_UNKNOWN_DEFAULT_OPEN_API StatusA = "unknown_default_open_api"
 )

 // All allowed values of StatusA enum
@@ -46,7 +46,7 @@ func (v *StatusA) UnmarshalJSON(src []byte) error {
                }
        }

-       *v = UNKNOWN_DEFAULT_OPEN_API
+       *v = STATUSA_UNKNOWN_DEFAULT_OPEN_API
        return nil
 }

will merge once all the tests pass

[wing328]

cc @antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04) @lwj5 (2023/04)

[wing328]

Thanks for the PR which has been merged.