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/render_test.go

@@ -2,6 +2,7 @@ package templatebuilder_test
 
 import (
 	"flag"
+	"fmt"
 	"os"
 	"path/filepath"
 	"testing"
@@ -14,6 +15,26 @@ import (
 
 var updateGolden = flag.Bool("update", false, "update golden files")
 
+// testRenderContext builds a BaseRenderContext from defaults and fills
+// in deterministic test values for any required variables that have no
+// default. This keeps the all-bases test loops working without silently
+// producing broken HCL.
+func testRenderContext(exampleID string) templatebuilder.BaseRenderContext {
+	rc := templatebuilder.DefaultBaseRenderContext(exampleID)
+	if rc.Variables == nil {
+		rc.Variables = make(map[string]string)
+	}
+	for _, v := range templatebuilder.BaseVariables(exampleID) {
+		if v.Computed || v.Sensitive {
+			continue
+		}
+		if _, ok := rc.Variables[v.Name]; !ok {
+			rc.Variables[v.Name] = fmt.Sprintf("%q", "test-"+v.Name)
+		}
+	}
+	return rc
+}
+
 func TestRenderBaseTemplate(t *testing.T) {
 	t.Parallel()
 
@@ -297,7 +318,7 @@ func TestAllBasesRenderAndExtractAgent(t *testing.T) {
 	for _, id := range templatebuilder.BaseTemplateIDs() {
 		t.Run(id, func(t *testing.T) {
 			t.Parallel()
-			renderCtx := templatebuilder.DefaultBaseRenderContext(id)
+			renderCtx := testRenderContext(id)
 			rendered, err := templatebuilder.RenderBaseTemplate(id, "main.tf.tmpl", renderCtx)
 			require.NoError(t, err, "base %q should render without error", id)
 			require.NotEmpty(t, rendered)
@@ -330,7 +351,7 @@ func TestBaseTemplateSnapshot(t *testing.T) {
 		t.Run(tc.exampleID, func(t *testing.T) {
 			t.Parallel()
 
-			renderCtx := templatebuilder.DefaultBaseRenderContext(tc.exampleID)
+			renderCtx := testRenderContext(tc.exampleID)
 			rendered, err := templatebuilder.RenderBaseTemplate(tc.exampleID, "main.tf.tmpl", renderCtx)
 			require.NoError(t, err)
 			require.NotEmpty(t, rendered)