feat(coderd/templatebuilder): add exampleID->OS map and base template .tf.tmpl files (#26115)

Add the bundled `exampleID -> OS` Go map for Docker, Kubernetes, and AWS
EC2 Linux base templates. Create `.tf.tmpl` Go template files for each
within `coderd/templatebuilder/bases/`, along with `BaseRenderContext`
and `RenderBaseTemplate` rendering helpers.

The `.tf.tmpl` files are independent copies of the example templates
with module blocks (code-server, jetbrains) removed, since the template
builder composes modules separately into `modules.tf`. When
`ImageOptions` is provided, the container image field references the
Terraform parameter; otherwise it uses the hardcoded value via Go
template whitespace control (`{{-`).

Golden file snapshot tests verify rendered output stability with an
`-update` flag for regeneration.

Depends on #25909

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

coderd/templatebuilder/render_test.go

@@ -0,0 +1,108 @@
+package templatebuilder_test
+
+import (
+	"flag"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/coderd/templatebuilder"
+)
+
+var updateGolden = flag.Bool("update", false, "update golden files")
+
+func TestRenderBaseTemplate(t *testing.T) {
+	t.Parallel()
+
+	t.Run("UnknownBase", func(t *testing.T) {
+		t.Parallel()
+		_, err := templatebuilder.RenderBaseTemplate("nonexistent", "main.tf.tmpl", templatebuilder.BaseRenderContext{})
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "unknown base template")
+	})
+
+	t.Run("InvalidPath", func(t *testing.T) {
+		t.Parallel()
+		_, err := templatebuilder.RenderBaseTemplate("docker", "nonexistent.tf.tmpl", templatebuilder.BaseRenderContext{})
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "not found")
+	})
+
+	imageOpts := []templatebuilder.ImageOption{
+		{Name: "Ubuntu", Value: "codercom/enterprise-base:ubuntu"},
+		{Name: "Custom", Value: "custom/image:latest"},
+	}
+
+	t.Run("DockerWithImageOptions", func(t *testing.T) {
+		t.Parallel()
+
+		renderCtx := templatebuilder.BaseRenderContext{
+			ContainerImage: "custom/image:latest",
+			ImageOptions:   imageOpts,
+		}
+		out, err := templatebuilder.RenderBaseTemplate("docker", "main.tf.tmpl", renderCtx)
+		require.NoError(t, err)
+		rendered := string(out)
+		require.Contains(t, rendered, `data.coder_parameter.container_image.value`)
+		require.Contains(t, rendered, `name  = "Ubuntu"`)
+		require.Contains(t, rendered, `name  = "Custom"`)
+		require.Contains(t, rendered, `coder_parameter`)
+	})
+
+	t.Run("KubernetesWithImageOptions", func(t *testing.T) {
+		t.Parallel()
+
+		renderCtx := templatebuilder.BaseRenderContext{
+			ContainerImage: "custom/image:latest",
+			ImageOptions:   imageOpts,
+		}
+		out, err := templatebuilder.RenderBaseTemplate("kubernetes", "main.tf.tmpl", renderCtx)
+		require.NoError(t, err)
+		rendered := string(out)
+		require.Contains(t, rendered, `data.coder_parameter.container_image.value`)
+		require.Contains(t, rendered, `name  = "Ubuntu"`)
+		require.Contains(t, rendered, `coder_parameter`)
+	})
+}
+
+func TestBaseTemplateSnapshot(t *testing.T) {
+	t.Parallel()
+
+	// This test table must cover every known base template.
+	// BaseTemplateIDs() is the source of truth; this list must match.
+	tests := []struct {
+		exampleID string
+	}{
+		{exampleID: "docker"},
+		{exampleID: "kubernetes"},
+		{exampleID: "aws-linux"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.exampleID, func(t *testing.T) {
+			t.Parallel()
+
+			renderCtx := templatebuilder.DefaultBaseRenderContext(tc.exampleID)
+			rendered, err := templatebuilder.RenderBaseTemplate(tc.exampleID, "main.tf.tmpl", renderCtx)
+			require.NoError(t, err)
+			require.NotEmpty(t, rendered)
+
+			goldenPath := filepath.Join("testdata", tc.exampleID+".tf.golden")
+
+			if *updateGolden {
+				err := os.MkdirAll("testdata", 0o755)
+				require.NoError(t, err)
+				err = os.WriteFile(goldenPath, rendered, 0o600)
+				require.NoError(t, err)
+				return
+			}
+
+			expected, err := os.ReadFile(goldenPath)
+			require.NoError(t, err, "golden file %s not found; run with -update to create", goldenPath)
+			require.Equal(t, string(expected), string(rendered),
+				"rendered output for %s does not match golden file; run with -update to regenerate", tc.exampleID)
+		})
+	}
+}