fix(coderd): start test AI bridge after config in automatic title tests (#27971)

Fixes the reopened flake tracked in CODAGT-876 / coder/internal#1629.

## Problem

The Aug 10 recurrence was not the previously fixed subtests failing
again: the nightly-gauntlet macos run ([job
log](https://github.com/coder/coder/actions/runs/31355808516/job/93355150162))
flaked in the sibling test
`TestPostChats_AutomaticTitleGenerationPasteOnly`, which the flake
investigator matched to the existing issue.

Both `TestPostChats_AutomaticTitleGeneration*` tests still used
`newChatClientWithAPI`, which starts the in-process AI Gateway daemon
before the test creates its provider/model config. The daemon's
synchronous initial provider load therefore sees zero providers, and
route availability depends on the asynchronous pubsub-driven reload
racing the one-shot automatic title generation that `CreateChat` kicks
off. When the reload loses (initial load at `.575` with
`provider_count=0`, title request at `.907` hitting `route not
supported`, reload landing at `1.029`), the title candidate fails
without retry and the test times out waiting for the `propose_title`
request.

## Fix

Convert both tests to the pattern #27564 established for the
`TestRegenerateChatTitle`/`TestProposeChatTitle` subtests:
`newChatClientWithoutAIBridge`, create the model config, then
`aibridgedtest.StartTestAIBridgeDaemon`, so the daemon's synchronous
initial load already contains the route. The existing `NoPubsubDelivery`
subtest guards that initial-load invariant, so no new guard test is
added.

## Validation

- Deterministic red-green via the isolated-pubsub technique: old
ordering with the pubsub leg removed reproduces the exact CI signature
(`provider_count=0`, `route not supported`, timeout at the
`titleRequested` wait); the new ordering passes with the pubsub leg
still removed, proving the synchronous initial load alone provides the
route.
- `go test ./coderd -run 'TestPostChats_AutomaticTitleGeneration'
-count=10`
- `go test ./coderd -run 'TestRegenerateChatTitle|TestProposeChatTitle'
-count=1`
- `make lint` and `make pre-commit` via hooks.

> Opened by Mux on Mike's behalf.

coderd/exp_chats_test.go

@@ -9914,9 +9914,10 @@ func TestPostChats_AutomaticTitleGeneration(t *testing.T) {
 		return chattest.OpenAINonStreamingResponse(`{"title": "Generated Title"}`)
 	})
 
-	client, api := newChatClientWithAPI(t)
+	client, _, api := newChatClientWithoutAIBridge(t)
 	firstUser := coderdtest.CreateFirstUser(t, client.Client)
 	_ = createChatModelConfigWithBaseURL(t, client, baseURL)
+	aibridgedtest.StartTestAIBridgeDaemon(t.Context(), t, api, nil)
 
 	chat, err := client.CreateChat(ctx, codersdk.CreateChatRequest{
 		OrganizationID: firstUser.OrganizationID,
@@ -9969,9 +9970,10 @@ func TestPostChats_AutomaticTitleGenerationPasteOnly(t *testing.T) {
 		return chattest.OpenAINonStreamingResponse(`{"title": "Generated Title"}`)
 	})
 
-	client, api := newChatClientWithAPI(t)
+	client, _, api := newChatClientWithoutAIBridge(t)
 	firstUser := coderdtest.CreateFirstUser(t, client.Client)
 	_ = createChatModelConfigWithBaseURL(t, client, baseURL)
+	aibridgedtest.StartTestAIBridgeDaemon(t.Context(), t, api, nil)
 
 	uploadResp, err := client.UploadChatFile(
 		ctx,