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.

codersdk/templatebuilder.go

@@ -1,6 +1,10 @@
 package codersdk
 
-import "encoding/json"
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+)
 
 // TemplateBuilderVariableType enumerates the variable types
 // supported by template builder module manifests.
@@ -40,3 +44,33 @@ type TemplateBuilderModule struct {
 type TemplateBuilderModulesResponse struct {
 	Modules []TemplateBuilderModule `json:"modules"`
 }
+
+// TemplateBuilderBase is the API response type for a base template
+// returned by GET /api/v2/templatebuilder/bases.
+type TemplateBuilderBase struct {
+	ID          string `json:"id"`
+	Name        string `json:"name"`
+	Description string `json:"description"`
+	Icon        string `json:"icon"`
+	OS          string `json:"os"`
+}
+
+// TemplateBuilderBasesResponse is the response body for listing template builder bases.
+type TemplateBuilderBasesResponse struct {
+	Bases []TemplateBuilderBase `json:"bases"`
+}
+
+// TemplateBuilderBases returns the list of base templates available
+// in the template builder.
+func (c *Client) TemplateBuilderBases(ctx context.Context) (TemplateBuilderBasesResponse, error) {
+	res, err := c.Request(ctx, http.MethodGet, "/api/v2/templatebuilder/bases", nil)
+	if err != nil {
+		return TemplateBuilderBasesResponse{}, err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return TemplateBuilderBasesResponse{}, ReadBodyAsError(res)
+	}
+	var resp TemplateBuilderBasesResponse
+	return resp, json.NewDecoder(res.Body).Decode(&resp)
+}