Follow-up to #1187, which typed every fmt.Errorf inside every Validate function in the openapi3 package. During that sweep I noted adjacent untyped sites; this records the ones that are genuinely spec Validate() errors and could benefit from typed clusters in a future PR. Not a prioritization signal.

Scope (trimmed). This now covers only errors raised from Validate() on the spec. An earlier version of this issue also listed loader $ref errors (loader.go), the JSON-Schema-compilation and value-validation paths (schema_jsonschema_validator.go, the schema.go value checks), and the NewRegexpFormatValidator constructor panic. Those are dropped: they fire during loading, during runtime value validation, or (for the constructor) at setup time, not during spec validation, so they never surface as Validate() findings and a typed cluster there wouldn't be reachable via the Validate() errors.As walk.

Candidates (spec validation only)

Each entry: current bare error → proposed typed cluster.

• Duplicate value in enum: [...] → EnumDuplicateValueError{Value}
• Duplicate entry in required: [...] → RequiredFieldDuplicatedError{Field}
• oneOf / anyOf / allOf element validation failure → SchemaCombinatorElementValidationError{Combinator, Cause} (context wrapper)
• tags element validation failure → TagInvalidError{Cause} (context wrapper)

Notes

• All Error() strings can be preserved byte-for-byte, following the same backward-compat pattern as openapi3: type the remaining bare-error validation sites #1187.
• The four-category model (Base / Cluster / Leaf / Context wrapper) documented in validation_error.go and validation_error_context.go applies.
• No ordering implied; these are candidates, ranked however the maintainers prefer.