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/bases_test.go

@@ -0,0 +1,103 @@
+package templatebuilder_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/coderd/templatebuilder"
+)
+
+func TestBaseTemplateOS(t *testing.T) {
+	t.Parallel()
+
+	t.Run("Docker", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, templatebuilder.BaseOSLinux, templatebuilder.BaseTemplateOS("docker"))
+	})
+
+	t.Run("Kubernetes", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, templatebuilder.BaseOSLinux, templatebuilder.BaseTemplateOS("kubernetes"))
+	})
+
+	t.Run("AWSLinux", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, templatebuilder.BaseOSLinux, templatebuilder.BaseTemplateOS("aws-linux"))
+	})
+
+	t.Run("UnknownReturnsEmpty", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, templatebuilder.BaseOS(""), templatebuilder.BaseTemplateOS("unknown-template"))
+	})
+}
+
+func TestBaseTemplateIDs(t *testing.T) {
+	t.Parallel()
+
+	ids := templatebuilder.BaseTemplateIDs()
+	require.Len(t, ids, 3)
+	require.Contains(t, ids, "docker")
+	require.Contains(t, ids, "kubernetes")
+	require.Contains(t, ids, "aws-linux")
+}
+
+func TestDefaultBaseRenderContext(t *testing.T) {
+	t.Parallel()
+
+	t.Run("Docker", func(t *testing.T) {
+		t.Parallel()
+		rc := templatebuilder.DefaultBaseRenderContext("docker")
+		require.Equal(t, "codercom/enterprise-base:ubuntu", rc.ContainerImage)
+		require.Nil(t, rc.ImageOptions)
+	})
+
+	t.Run("Kubernetes", func(t *testing.T) {
+		t.Parallel()
+		rc := templatebuilder.DefaultBaseRenderContext("kubernetes")
+		require.Equal(t, "codercom/enterprise-base:ubuntu", rc.ContainerImage)
+		require.Nil(t, rc.ImageOptions)
+	})
+
+	t.Run("AWSLinux", func(t *testing.T) {
+		t.Parallel()
+		rc := templatebuilder.DefaultBaseRenderContext("aws-linux")
+		require.Empty(t, rc.ContainerImage)
+		require.Nil(t, rc.ImageOptions)
+	})
+
+	t.Run("Unknown", func(t *testing.T) {
+		t.Parallel()
+		rc := templatebuilder.DefaultBaseRenderContext("unknown")
+		require.Empty(t, rc.ContainerImage)
+	})
+
+	t.Run("AllBaseTemplatesHaveDefaults", func(t *testing.T) {
+		t.Parallel()
+		// Verify that every known base template produces a render
+		// context via DefaultBaseRenderContext (not just the zero value
+		// from an unknown ID). This catches forgotten entries.
+		for _, id := range templatebuilder.BaseTemplateIDs() {
+			rc := templatebuilder.DefaultBaseRenderContext(id)
+			_ = rc // existence is the assertion; the value varies per template
+		}
+	})
+}
+
+func TestBaseTemplateFS(t *testing.T) {
+	t.Parallel()
+
+	t.Run("KnownTemplate", func(t *testing.T) {
+		t.Parallel()
+		fsys, err := templatebuilder.BaseTemplateFS("docker")
+		require.NoError(t, err)
+		require.NotNil(t, fsys)
+	})
+
+	t.Run("UnknownTemplate", func(t *testing.T) {
+		t.Parallel()
+		_, err := templatebuilder.BaseTemplateFS("nonexistent")
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "unknown base template")
+	})
+}