fix(backend): remove initialPrompt skip that blocked display name fallback (#1562)

## Summary

- **Removed** the 5-line `initialPrompt` skip guard in
`agui_proxy.go:1005-1009` that prevented the AG-UI proxy from acting as
a fallback when CreateSession's async display name generation failed
- **Updated** the comment in `sessions.go` to document the new fallback
relationship between the two generation paths
- **Added** 4 regression tests for
`triggerDisplayNameGenerationIfNeeded` covering the key code paths

## Problem

Sessions intermittently showed raw session IDs (`session-<UUID>`)
instead of friendly display names. The display name system has two
generation paths:

- **Path A** (CreateSession handler): fires `GenerateDisplayNameAsync`
in a goroutine with 10s timeout
- **Path B** (AG-UI proxy): generates on first `/agui/run` POST

Path B intentionally skipped messages matching `initialPrompt` to
prevent double-generation. But this also blocked Path B from acting as a
fallback when Path A failed (API timeout, Vertex errors, missing
credentials). The only recovery was the user sending a second message.

## Fix

The `initialPrompt` skip was **redundant** — two existing guards already
prevent double-generation:

1. `ShouldGenerateDisplayName(spec)` returns `false` if `displayName` is
already set
2. `updateSessionDisplayNameInternal` has a first-write-wins race guard
at `display_name.go:302-307`

Removing the skip lets Path B naturally retry when Path A fails, with no
risk of overwrites. Worst case in a race: one redundant Haiku API call
(~$0.001).

## Test plan

- [x] All 26 websocket package tests pass (including 4 new regression
tests)
- [x] `gofmt` and `go vet` clean
- [ ] Verify in staging: create sessions with `initialPrompt`, confirm
display name appears even when Vertex API is slow/unavailable

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved display name generation to be more consistent and reliable
when creating sessions with initial prompts.

* **Tests**
* Added comprehensive test coverage for display name generation
scenarios, including edge cases.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: 3 people

components/backend/handlers/display_name.go

@@ -80,7 +80,7 @@ func ValidateDisplayName(name string) string {
 // - Gracefully handles session deletion during generation (checks IsNotFound)
 // - No cancellation mechanism exists; goroutine runs to completion or timeout
 // - Safe for backend restarts: orphaned goroutines will timeout naturally
-func GenerateDisplayNameAsync(projectName, sessionName, userMessage string, sessionCtx SessionContext) {
+var GenerateDisplayNameAsync = func(projectName, sessionName, userMessage string, sessionCtx SessionContext) {
 	go func() {
 		defer func() {
 			if r := recover(); r != nil {

components/backend/handlers/sessions.go

@@ -1203,9 +1203,8 @@ func CreateSession(c *gin.Context) {
 	// This ensures consistent behavior whether sessions are created via API or kubectl.
 
 	// Trigger async display name generation when initialPrompt is provided
-	// but no explicit displayName was set. The AG-UI proxy skips the
-	// initialPrompt message, so sessions created with only an initialPrompt
-	// (e.g., from the new-session page) would never get a generated name.
+	// but no explicit displayName was set. The AG-UI proxy also generates
+	// on the first /agui/run message as a fallback if this call fails.
 	if strings.TrimSpace(req.InitialPrompt) != "" && strings.TrimSpace(req.DisplayName) == "" {
 		spec, ok := created.Object["spec"].(map[string]interface{})
 		if ok {

components/backend/websocket/agui_proxy.go

@@ -1002,12 +1002,6 @@ func triggerDisplayNameGenerationIfNeeded(projectName, sessionName string, messa
 		return
 	}
 
-	// Skip if this message is the auto-sent initialPrompt
-	initialPrompt, _, _ := unstructured.NestedString(spec, "initialPrompt")
-	if initialPrompt != "" && strings.TrimSpace(userMessage) == strings.TrimSpace(initialPrompt) {
-		return
-	}
-
 	if !handlers.ShouldGenerateDisplayName(spec) {
 		return
 	}

components/backend/websocket/agui_proxy_test.go

@@ -1,15 +1,21 @@
 package websocket
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
 
 	"ambient-code-backend/handlers"
+	"ambient-code-backend/tests/test_utils"
+	"ambient-code-backend/types"
 
 	corev1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/client-go/dynamic"
 	k8sfake "k8s.io/client-go/kubernetes/fake"
 )
 
@@ -204,3 +210,129 @@ func TestDefaultRunnerPort_Constant(t *testing.T) {
 		t.Errorf("Expected DefaultRunnerPort=8001, got %d", handlers.DefaultRunnerPort)
 	}
 }
+
+// --- triggerDisplayNameGenerationIfNeeded tests (regression for #1561) ---
+
+func setupDisplayNameTest(t *testing.T, spec map[string]interface{}) (cleanup func()) {
+	t.Helper()
+
+	oldDynamic := handlers.DynamicClient
+	oldK8sProjects := handlers.K8sClientProjects
+	oldGVRFunc := handlers.GetAgenticSessionV1Alpha1Resource
+
+	agenticSessionGVR := schema.GroupVersionResource{
+		Group:    "vteam.ambient-code",
+		Version:  "v1alpha1",
+		Resource: "agenticsessions",
+	}
+	handlers.GetAgenticSessionV1Alpha1Resource = func() schema.GroupVersionResource {
+		return agenticSessionGVR
+	}
+
+	fakeClients := test_utils.NewFakeClientSet()
+	handlers.DynamicClient = fakeClients.GetDynamicClient()
+	handlers.K8sClientProjects = fakeClients.GetK8sClient()
+
+	err := test_utils.CreateAgenticSessionInFakeClient(
+		handlers.DynamicClient, "test-project", "test-session", spec,
+	)
+	if err != nil {
+		t.Fatalf("Failed to create test session: %v", err)
+	}
+
+	return func() {
+		handlers.DynamicClient = oldDynamic
+		handlers.K8sClientProjects = oldK8sProjects
+		handlers.GetAgenticSessionV1Alpha1Resource = oldGVRFunc
+	}
+}
+
+func getDisplayName(t *testing.T, dc dynamic.Interface) string {
+	t.Helper()
+	gvr := handlers.GetAgenticSessionV1Alpha1Resource()
+	item, err := dc.Resource(gvr).Namespace("test-project").Get(
+		context.Background(), "test-session", metav1.GetOptions{},
+	)
+	if err != nil {
+		t.Fatalf("Failed to get session: %v", err)
+	}
+	dn, _, _ := unstructured.NestedString(item.Object, "spec", "displayName")
+	return dn
+}
+
+func TestTriggerDisplayName_InitialPromptNotSkipped(t *testing.T) {
+	cleanup := setupDisplayNameTest(t, map[string]interface{}{
+		"initialPrompt": "Help me debug auth",
+	})
+	defer cleanup()
+
+	called := false
+	oldFn := handlers.GenerateDisplayNameAsync
+	handlers.GenerateDisplayNameAsync = func(projectName, sessionName, userMessage string, sessionCtx handlers.SessionContext) {
+		called = true
+	}
+	defer func() { handlers.GenerateDisplayNameAsync = oldFn }()
+
+	msgs := []types.Message{
+		{ID: "msg-1", Role: "user", Content: "Help me debug auth"},
+	}
+
+	triggerDisplayNameGenerationIfNeeded("test-project", "test-session", msgs)
+
+	if !called {
+		t.Error("Expected GenerateDisplayNameAsync to be called for initialPrompt message when displayName is empty")
+	}
+}
+
+func TestTriggerDisplayName_SkipsWhenNameAlreadySet(t *testing.T) {
+	cleanup := setupDisplayNameTest(t, map[string]interface{}{
+		"initialPrompt": "Help me debug auth",
+		"displayName":   "Debug Auth Middleware",
+	})
+	defer cleanup()
+
+	msgs := []types.Message{
+		{ID: "msg-1", Role: "user", Content: "Help me debug auth"},
+	}
+
+	triggerDisplayNameGenerationIfNeeded("test-project", "test-session", msgs)
+
+	// displayName should remain unchanged — ShouldGenerateDisplayName
+	// returns false when displayName is already set.
+	dn := getDisplayName(t, handlers.DynamicClient)
+	if dn != "Debug Auth Middleware" {
+		t.Errorf("Expected displayName to remain %q, got %q", "Debug Auth Middleware", dn)
+	}
+}
+
+func TestTriggerDisplayName_SkipsWhenNoUserMessage(t *testing.T) {
+	cleanup := setupDisplayNameTest(t, map[string]interface{}{
+		"initialPrompt": "Help me debug auth",
+	})
+	defer cleanup()
+
+	// Only assistant messages — no user content to generate from
+	msgs := []types.Message{
+		{ID: "msg-1", Role: "assistant", Content: "I'll help you debug"},
+	}
+
+	triggerDisplayNameGenerationIfNeeded("test-project", "test-session", msgs)
+
+	dn := getDisplayName(t, handlers.DynamicClient)
+	if dn != "" {
+		t.Errorf("Expected empty displayName, got %q", dn)
+	}
+}
+
+func TestTriggerDisplayName_SkipsWhenDynamicClientNil(t *testing.T) {
+	oldDynamic := handlers.DynamicClient
+	handlers.DynamicClient = nil
+	defer func() { handlers.DynamicClient = oldDynamic }()
+
+	msgs := []types.Message{
+		{ID: "msg-1", Role: "user", Content: "Help me debug auth"},
+	}
+
+	// Should return early without panic when DynamicClient is nil
+	triggerDisplayNameGenerationIfNeeded("test-project", "test-session", msgs)
+}