`portkey_api_key`: "inconsistent result after apply" on `last_reset_at` when adding `usage_limits`

[atrendler-tc]

Provider version: 0.2.28
Terraform version: 1.14.x

What happened

Adding a usage_limits block (with periodic_reset) to an existing portkey_api_key that previously had no limits fails at apply:

Error: Provider produced inconsistent result after apply

When applying changes to portkey_api_key.example, provider produced an
unexpected new value: .last_reset_at: was null, but now
cty.StringVal("2026-07-23T21:13:06Z").

The API write actually succeeds. The error fires after it and re-running apply then reconciles (which is how we've been working around it).

Steps to reproduce

1. Create a portkey_api_key without a usage_limits block. last_reset_at is null in state.
2. Add a usage_limits block that includes periodic_reset.
3. terraform apply.

# Step 1 — apply this first (no usage_limits)
resource "portkey_api_key" "example" {
  name         = "example-app"
  type         = "workspace"
  sub_type     = "service"
  workspace_id = var.workspace_id
}

# Step 2 — add usage_limits, then apply again → error
resource "portkey_api_key" "example" {
  name         = "example-app"
  type         = "workspace"
  sub_type     = "service"
  workspace_id = var.workspace_id

  usage_limits = {
    type            = "cost"
    credit_limit    = 100
    alert_threshold = 80
    periodic_reset  = "monthly"
  }
}

Expected

Apply succeeds; last_reset_at reflects the API-returned timestamp.

Actual

Provider produced inconsistent result after apply on last_reset_at.