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/compose_test.go

@@ -237,7 +237,8 @@ func TestCompose(t *testing.T) {
 			},
 		})
 		require.Error(t, err)
-		require.Contains(t, err.Error(), "render module")
+		require.Contains(t, err.Error(), `variable "url"`)
+		require.Contains(t, err.Error(), "is required")
 	})
 }