test: cover grpc-transcode empty array responses

[bytelazy]

Description

In the grpc-transcode plugin, when a repeated field in the gRPC response is empty, it is incorrectly encoded as an empty JSON object ({}) instead of an empty JSON array ([]). This behavior does not comply with the gRPC-JSON transcoding specification and can cause type-related errors on the client-side, which expects an array.

Fixes #11440

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the [APISIX mailing list](https://github.com/apache/apisix/tree/master#community) first)

Changes or the new features added to this PR
The main changes are in the response.lua file to correctly handle the serialization of repeated fields.

Identify repeated fields: A new function fetch_proto_array_names has been added. It recursively traverses the loaded Protobuf schema definition to identify and collect the names of all fields marked as repeated.

Ensure correct JSON array serialization:

A new function set_default_array has been introduced. After the gRPC response is decoded into a Lua table, this function traverses the table.

For any field that was identified as repeated, it sets a specific metatable (core.json.array_mt).

This metatable forces the core.json.encode function to serialize the corresponding Lua table as a JSON array ([]), even when it is empty.

This logic is applied just before the decoded response is encoded into the final JSON output, ensuring the structure is correct.