feat: add the /aitasks/prompts endpoint (#18464)

Add an endpoint to fetch AI task prompts for multiple workspace builds
at the same time. A prompt is the value of the "AI Prompt" workspace
build parameter. On main, the only way our API allows fetching workspace
build parameters is by using the `/workspacebuilds/$build_id/parameters`
endpoint, requiring a separate API call for every build.

The Tasks dashboard fetches Task workspaces in order to show them in a
list, and then needs to fetch the value of the `AI Prompt` parameter for
every task workspace (using its latest build id), requiring an
additional API call for each list item. This endpoint will allow the
dashboard to make just 2 calls to render the list: one to fetch task
workspaces, the other to fetch prompts.

<img width="1512" alt="Screenshot 2025-06-20 at 11 33 11"
src="https://github.com/user-attachments/assets/92899999-e922-44c5-8325-b4b23a0d2bff"
/>

Related to https://github.com/coder/internal/issues/660.

coderd/aitasks.go

@@ -0,0 +1,63 @@
+package coderd
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/google/uuid"
+
+	"github.com/coder/coder/v2/coderd/httpapi"
+	"github.com/coder/coder/v2/codersdk"
+)
+
+// This endpoint is experimental and not guaranteed to be stable, so we're not
+// generating public-facing documentation for it.
+func (api *API) aiTasksPrompts(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	buildIDsParam := r.URL.Query().Get("build_ids")
+	if buildIDsParam == "" {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "build_ids query parameter is required",
+		})
+		return
+	}
+
+	// Parse build IDs
+	buildIDStrings := strings.Split(buildIDsParam, ",")
+	buildIDs := make([]uuid.UUID, 0, len(buildIDStrings))
+	for _, idStr := range buildIDStrings {
+		id, err := uuid.Parse(strings.TrimSpace(idStr))
+		if err != nil {
+			httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+				Message: fmt.Sprintf("Invalid build ID format: %s", idStr),
+				Detail:  err.Error(),
+			})
+			return
+		}
+		buildIDs = append(buildIDs, id)
+	}
+
+	parameters, err := api.Database.GetWorkspaceBuildParametersByBuildIDs(ctx, buildIDs)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error fetching workspace build parameters.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	promptsByBuildID := make(map[string]string, len(parameters))
+	for _, param := range parameters {
+		if param.Name != codersdk.AITaskPromptParameterName {
+			continue
+		}
+		buildID := param.WorkspaceBuildID.String()
+		promptsByBuildID[buildID] = param.Value
+	}
+
+	httpapi.Write(ctx, rw, http.StatusOK, codersdk.AITasksPromptsResponse{
+		Prompts: promptsByBuildID,
+	})
+}