fix(coderd): declare project_id variable in GCP template builder bases (#27015)

## Summary

GCP base templates (`gcp-linux`, `gcp-windows`) in the Template Builder
had a Terraform `variable "project_id"` with no default, but their
`base.json` manifests didn't declare it. The UI never prompted for it,
so the provisioner import always failed with:

```
required template variables need values: project_id
```

## Changes

- Add `project_id` as a required variable in both GCP `base.json`
manifests
- Convert templates from raw Terraform variable blocks to Go template
injection (`{{ .Variables.project_id }}`), matching the existing
kubernetes pattern
- Fix `DefaultBaseRenderContext` to supply a `"REQUIRED"` placeholder
for required variables without defaults (previously rendered as `<no
value>`)
- Replace duplicate test subtests with proper GCP coverage including a
missing-variable error case

<details>
<summary>Implementation plan</summary>

### Root cause

The GCP base templates contained `variable "project_id" {}` (no default
= required) in their `.tf.tmpl` files, but the `base.json` manifests had
an empty `variables` array. The Template Builder UI
(`BaseTemplateParametersStep`) is data-driven from `base.json`, so it
never showed a field for `project_id`. The composed Terraform output
still contained the required variable, causing the provisioner import to
fail.

### Fix approach

Follow the pattern established by the kubernetes base template:
1. Declare variables in `base.json` so the UI prompts for them
2. Use Go template syntax (`{{ .Variables.project_id }}`) to inject
values at compose time
3. Remove raw Terraform `variable` blocks from the template since the
value is now baked in

### Files changed

| File | Change |
|---|---|
| `bases/gcp-linux/base.json` | Added `project_id` as a required
variable |
| `bases/gcp-windows/base.json` | Added `project_id` as a required
variable |
| `bases/gcp-linux/main.tf.tmpl` | Removed Terraform variable block, use
Go template injection |
| `bases/gcp-windows/main.tf.tmpl` | Same |
| `bases.go` | `DefaultBaseRenderContext` supplies placeholder for
required vars without defaults |
| `compose_test.go` | Replaced duplicate subtests with proper GCP tests
|
| `templatebuilder_handler_test.go` | Updated `gcp-windows` spec to
expect `project_id` variable |
| Golden files | Regenerated |

</details>

> 🤖 Generated by Coder Agents on behalf of @jeremyruppel

coderd/templatebuilder/compose_test.go

@@ -78,36 +78,44 @@ func TestCompose(t *testing.T) {
 		require.Contains(t, result.ExtraFiles, "cloud-init/userdata.sh.tftpl")
 	})
 
-	t.Run("GCPWindowsBase", func(t *testing.T) {
+	t.Run("GCPLinuxBaseWithProjectID", func(t *testing.T) {
 		t.Parallel()
 		result, err := templatebuilder.Compose(templatebuilder.ComposeRequest{
-			BaseTemplateID: "gcp-windows",
-			RegistryURL:    "https://registry.coder.com",
+			BaseTemplateID:     "gcp-linux",
+			BaseVariableValues: map[string]string{"project_id": "my-gcp-project"},
+			RegistryURL:        "https://registry.coder.com",
 		})
 		require.NoError(t, err)
 		require.NotEmpty(t, result.MainTF)
-		require.Contains(t, string(result.MainTF), `resource "coder_agent" "main"`)
-	})
-
-	t.Run("AzureLinuxExtraFiles", func(t *testing.T) {
-		t.Parallel()
-		result, err := templatebuilder.Compose(templatebuilder.ComposeRequest{
-			BaseTemplateID: "azure-linux",
-			RegistryURL:    "https://registry.coder.com",
-		})
-		require.NoError(t, err)
-		require.Contains(t, result.ExtraFiles, "cloud-init/cloud-config.yaml.tftpl")
+		mainTF := string(result.MainTF)
+		require.Contains(t, mainTF, `resource "coder_agent" "main"`)
+		require.Contains(t, mainTF, `default     = "my-gcp-project"`)
+		require.Contains(t, mainTF, `project = var.project_id`)
 	})
 
 	t.Run("GCPWindowsBase", func(t *testing.T) {
 		t.Parallel()
 		result, err := templatebuilder.Compose(templatebuilder.ComposeRequest{
-			BaseTemplateID: "gcp-windows",
-			RegistryURL:    "https://registry.coder.com",
+			BaseTemplateID:     "gcp-windows",
+			BaseVariableValues: map[string]string{"project_id": "my-gcp-project"},
+			RegistryURL:        "https://registry.coder.com",
 		})
 		require.NoError(t, err)
 		require.NotEmpty(t, result.MainTF)
-		require.Contains(t, string(result.MainTF), `resource "coder_agent" "main"`)
+		mainTF := string(result.MainTF)
+		require.Contains(t, mainTF, `resource "coder_agent" "main"`)
+		require.Contains(t, mainTF, `default     = "my-gcp-project"`)
+		require.Contains(t, mainTF, `project = var.project_id`)
+	})
+
+	t.Run("GCPMissingRequiredBaseVariable", func(t *testing.T) {
+		t.Parallel()
+		_, err := templatebuilder.Compose(templatebuilder.ComposeRequest{
+			BaseTemplateID: "gcp-linux",
+			RegistryURL:    "https://registry.coder.com",
+		})
+		require.Error(t, err)
+		require.Contains(t, err.Error(), `variable "project_id" is required`)
 	})
 
 	t.Run("AzureLinuxExtraFiles", func(t *testing.T) {