feat: add GET /api/v2/templatebuilder/bases endpoint (#26116)

Implement `GET /api/v2/templatebuilder/bases`, which returns the list of
base templates available in the template builder. Reads from the bundled
catalog by cross-referencing `templatebuilder.BaseTemplateIDs()` with
`examples.List()`, enriching each entry with the OS from the `exampleID
-> OS` map.

The endpoint is gated behind the template builder feature flag (returns
404 when disabled) and requires `policy.ActionRead` on
`rbac.ResourceTemplate`.

Depends on #26115

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

coderd/templatebuilder_handler_test.go

@@ -0,0 +1,83 @@
+package coderd_test
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/coderd/coderdtest"
+	"github.com/coder/coder/v2/coderd/templatebuilder"
+	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/testutil"
+)
+
+func TestTemplateBuilderBases(t *testing.T) {
+	t.Parallel()
+
+	t.Run("OK", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		resp, err := client.TemplateBuilderBases(ctx)
+		require.NoError(t, err)
+		require.NotEmpty(t, resp.Bases)
+		require.Len(t, resp.Bases, len(templatebuilder.BaseTemplateIDs()))
+
+		basesByID := make(map[string]codersdk.TemplateBuilderBase, len(resp.Bases))
+		for _, b := range resp.Bases {
+			basesByID[b.ID] = b
+		}
+
+		for _, id := range templatebuilder.BaseTemplateIDs() {
+			b, ok := basesByID[id]
+			require.True(t, ok, "base %q missing from response", id)
+			require.NotEmpty(t, b.Name)
+			require.NotEmpty(t, b.Icon)
+			require.Equal(t, string(templatebuilder.BaseTemplateOS(id)), b.OS)
+		}
+	})
+
+	t.Run("Sorted", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		resp, err := client.TemplateBuilderBases(ctx)
+		require.NoError(t, err)
+
+		for i := 1; i < len(resp.Bases); i++ {
+			require.LessOrEqual(t, resp.Bases[i-1].Name, resp.Bases[i].Name,
+				"bases should be sorted by name")
+		}
+	})
+
+	t.Run("DisabledReturns404", func(t *testing.T) {
+		t.Parallel()
+		dv := coderdtest.DeploymentValues(t)
+		dv.TemplateBuilder.Disabled = true
+
+		client := coderdtest.New(t, &coderdtest.Options{
+			DeploymentValues: dv,
+		})
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		_, err := client.TemplateBuilderBases(ctx)
+		require.Error(t, err)
+
+		var sdkErr *codersdk.Error
+		require.ErrorAs(t, err, &sdkErr)
+		require.Equal(t, http.StatusNotFound, sdkErr.StatusCode())
+	})
+}