Handle multi-element arrays in flattenedEncoder and flattenedDecoder

[twingate-blee]

Summary

Fixes a bug where arrays/lists were truncated to only the first element in the Go SDK during Read operations. This only affected the Go SDK - Python, TypeScript, and .NET SDKs worked correctly.

Fixes: #3240

Problem

When using Plugin Framework providers with ListNestedBlock or SetNestedBlock types, the Go SDK would:

• ✅ Show all array elements after initial resource creation
• ❌ Truncate arrays to 1 element after any update (which triggers a Read operation)
• ✅ Work correctly in Python, TypeScript, and .NET SDKs

Example (Twingate provider)

Before fix:

{
  "inputs": {
    "accessServices": [2 elements],
    "accessGroups": [2 elements]
  },
  "outputs": {
    "accessServices": [1 element],  // ❌ Truncated!
    "accessGroups": [1 element]     // ❌ Truncated!
  }
}

After fix:

{
  "inputs": {
    "accessServices": [2 elements],
    "accessGroups": [2 elements]
  },
  "outputs": {
    "accessServices": [2 elements],  // ✅ Fixed!
    "accessGroups": [2 elements]     // ✅ Fixed!
  }
}

Root Cause

The bug occurred in two places in pkg/convert/flattened.go:

1. flattenedEncoder.fromPropertyValue() (lines 29-58): Only handled single elements, not arrays. When encoding multi-element arrays, only the first element was sent to the Terraform provider.

2. flattenedDecoder.toPropertyValue() (lines 58-74): Returned an error when receiving >1 elements. This error was swallowed somewhere in the chain, causing only the first element to be preserved in Go SDK outputs.

These encoders/decoders are selected when IsMaxItemsOne() incorrectly identifies a multi-item list as single-item. This commonly occurs with Plugin Framework providers using ListNestedBlock or SetNestedBlock.

The Fix

Encoder Fix

• Checks if input is an array using v.IsArray()
• If so, encodes all elements and returns them as a list
• Maintains backward compatibility for legitimate single-item inputs

Decoder Fix

• When list has >1 elements, decodes all into an array
• Returns as resource.NewArrayProperty(values) instead of erroring
• Maintains backward compatibility for 0 or 1 element cases

Testing

Tested with the Twingate Pulumi provider, which uses Plugin Framework ListNestedBlock types:

Test Results

Before fix:

• ✅ Initial create: Arrays show all elements (2)
• ❌ After update (triggers Read): Arrays truncated to 1 element
• ✅ Only affected Go SDK; Python/TypeScript/.NET worked correctly

After fix:

• ✅ Create operations: All elements preserved
• ✅ Update operations: All elements preserved
• ✅ Read/refresh operations: All elements preserved
• ✅ Works for both types.List and types.Set
• ✅ Works for both ListNestedBlock and SetNestedBlock
• ✅ Independent arrays remain independent

[guineveresaenger]

Hi @twingate-blee - thank you for opening this pull request. Let me run tests for you!