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

[kdelay]

Description

go/model_oneof.mustache renders the "no match" error return inside a {{#oneOf}} section, so the if err != nil { ... } else { ... } block added by #24349 is emitted once per oneOf member. Only the first copy can ever run; the remaining copies are dead code and grow with the member count.

Committed sample output shows it today, e.g. samples/openapi3/client/petstore/go/go-petstore/model_one_of_primitive_type.go (3 members, so 3 copies).

The fix renders the block once, outside the section. err is declared unconditionally as var err error at the top of UnmarshalJSON, so the {{^oneOf}} fallback that existed for the empty-oneOf case is no longer needed. The else is dropped in favour of an early return, which is what gofmt-style Go looks like here.

Behaviour is unchanged: the first (and only reachable) copy is exactly what the new single block renders. Both branches of the template (useOneOfDiscriminatorLookup on and off) had the same duplication and both are fixed.

fix #24661

Verification

• ./mvnw clean package -DskipTests — BUILD SUCCESS.

• ./bin/generate-samples.sh ./bin/configs/go-*.yaml — 21 generators, no unrelated churn; 8 sample files changed, all of them the duplicated block collapsing to one.

• Go generator unit tests: AbstractGoCodegenTest, GoClientCodegenTest, GoClientOptionsTest, GoModelTest — 53 tests, 0 failures.

• go build ./... and go vet ./... on the regenerated samples/openapi3/client/petstore/go/go-petstore and samples/client/others/go/oneof-anyof-required (golang:1.23) — both clean.

• Runtime check that the #24349 behaviour survives, against the regenerated petstore sample:

  payload true    -> data failed to match schemas in oneOf(OneOfPrimitiveType): json: cannot unmarshal bool into Go value of type int32
  payload "abc"   -> data failed to match schemas in oneOf(OneOfPrimitiveType): json: cannot unmarshal string into Go value of type int32
  

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/cc @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d @lwj5

---

Summary by cubic

Emit the oneOf no-match error once in generated Go UnmarshalJSON methods instead of once per member, removing unreachable duplicates as oneOf grows. Behavior is unchanged; regenerated Go samples now contain a single error block.

• Bug Fixes
  • Move the no-match error block outside the {{#oneOf}} section in model_oneof.mustache; drop the empty-oneOf fallback and replace else with an early return.
  • Apply the fix to both discriminator paths; update Go samples to collapse duplicated error code.

^{Written for commit 0050666. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 9 files

_{Re-trigger cubic}