Preview panics on Terraform `check` blocks with unresolved data sources

Problem

When a Terraform template uses a check block that references a data source (e.g. data "external" or data "docker_network"), workspace creation fails with:

Panic occurred in preview. This should not happen, please report this to Coder.
panic in preview: value is null

The template imports fine and terraform plan/apply handles the check block correctly, but coder create fails at the preview/parameter validation step.

Steps to reproduce

1. Create a template with a check block that references a data source:

check "docker_is_reachable" {
  data "external" "docker_check" {
    program = ["sh", "-c", "echo '{\"status\":\"ok\"}'"] 
  }

  assert {
    condition     = data.external.docker_check.result.status == "ok"
    error_message = "Docker is not reachable."
  }
}

2. Push the template with coder templates push (succeeds)
3. Run coder create against the template
4. Preview panics

Root cause

The Preview() function in preview.go uses trivy's HCL parser to evaluate the Terraform configuration. The parser can't execute providers, so data sources like data.external.docker_check resolve to a null cty.Value.

When the parser evaluates the check block's assert condition:

condition = data.external.docker_check.result.status == "ok"

accessing .result.status on the null value triggers a panic in go-cty at value_ops.go:1162:

if val.IsNull() {
    panic("value is null")
}

The recover() at preview.go:148 catches this and wraps it as the diagnostic the user sees.

Proposed fix

Terraform check blocks are non-blocking validation that runs as the last step of plan/apply. They have no bearing on parameter extraction, presets, tags, or any other preview concern. The preview should either:

1. Skip check blocks entirely during evaluation, or
2. Guard against null values from unresolved data sources when evaluating expressions inside check blocks

Option 1 seems cleanest since check blocks are irrelevant to the preview's purpose.

Context

This came up while adding a Docker connectivity check to Coder's starter Docker template. The check block verifies the Docker daemon is reachable and surfaces a clear warning with setup docs instead of a generic provider error. The check works perfectly in Terraform itself but breaks Coder's workspace creation flow.

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This issue was created by Coder Agents on behalf of @bpmct.