fix(goctl/swagger): emit items for nested interface arrays

[0xC3B6]

Fixes #5426

Summary

• Map API interface{} types to Swagger object schemas when building array item schemas.
• Preserve nested array item chains for shapes like [][]interface{} so generated Swagger 2.0 includes the required nested items field.

Test Plan

• GOWORK=/tmp/go-zero-swagger-work/go.work GOCACHE=/tmp/go-build GOPROXY=https://goproxy.cn,direct go test ./api/swagger

[kevwan]

Review

Clean and targeted fix for swagger generation of interface{} array types (fixes #5426).

Problem: itemFromGoType() had no case for apiSpec.InterfaceType, causing the function to return nil for []interface{} fields. This meant the generated Swagger had items: null, which is invalid per OpenAPI 2.0 spec.

Changes:

1. itemFromGoType — Added case apiSpec.InterfaceType that returns {type: "object"}. This is the correct representation: interface{} can be any JSON value, and object is the closest Swagger 2.0 approximation.

2. typeFromGoType / sampleTypeFromGoType — Added apiSpec.InterfaceType to the struct/map case, so these two helpers also return "object" for interface{} values. This ensures consistency across all three type-mapping functions.

Test: TestNestedArrayInterfaceItems validates a [][]interface{} field, confirming the nested array has correct items → items → {type: object} chain.

One note: Using "object" for interface{} is a pragmatic choice — the technically correct Swagger 2.0 representation would be to omit type constraints (empty schema {}), which matches any JSON type. However, "object" is widely accepted and consistent with how other generators handle this case.

LGTM — the fix is minimal, correct, and well-tested.