fix: render base variables into templates instead of tfvars (DEVEX-287) (#26436)

Part of the Template Builder wizard PR stack.

## Problem

The kubernetes base template used Terraform `variable` blocks and
`var.*` references for `use_kubeconfig` and `namespace`, but the
composed tar bundle never included a `.tfvars` file. This caused
`terraform plan` to fail with "required template variables need values:
namespace".

## Fix

Base templates now use Go template variables (`{{ .Variables.* }}`) just
like module templates do. Values are validated, HCL-quoted, and rendered
directly into the output HCL.

Also adds explicit "variable is required" validation to both
`mergeBaseVariables` and `mergeModuleVariables`, replacing the previous
reliance on `missingkey=error` at render time for clearer error
messages.

---
> [!NOTE]
> Generated by Coder Agents on behalf of @jeremyruppel

coderd/templatebuilder/render_test.go

@@ -58,6 +58,10 @@ func TestRenderBaseTemplate(t *testing.T) {
 		renderCtx := templatebuilder.BaseRenderContext{
 			ContainerImage: "custom/image:latest",
 			ImageOptions:   imageOpts,
+			Variables: map[string]string{
+				"namespace":      `"test-ns"`,
+				"use_kubeconfig": "false",
+			},
 		}
 		out, err := templatebuilder.RenderBaseTemplate("kubernetes", "main.tf.tmpl", renderCtx)
 		require.NoError(t, err)