test(coderd): centralize chat test harness and stabilize flakes (#25171)

Chat tests previously constructed a real `openai` provider with a fake
API key and no `BaseURL`, so background title generation hit
`api.openai.com` and timed out under `-race`. The same root cause
produced several distinct flakes: title regeneration races with
synchronous `UpdateChat`/`ProposeChatTitle`, and pagination races
against `updated_at` bumps from real-network processing.

This moves the fake OpenAI-compatible provider and the chat-settle wait
into first-class `coderdtest` capabilities.
`coderd.Options.ChatProviderAPIKeys` is the new seam tests use to
redirect chat traffic to a local `httptest.Server`.
`coderdtest.WaitForChatSettled` replaces per-test waiters and drains
tracked chat-daemon work after the chat row leaves `pending`/`running`.
The `newChatClient*` constructors funnel through one options builder
that installs the fake provider before the coderd test server so cleanup
ordering is deterministic.

Closes https://github.com/coder/internal/issues/1528 & Closes ENG-2659
Closes https://github.com/coder/internal/issues/1480 & Closes CODAGT-359
Closes https://github.com/coder/internal/issues/1507 & Closes CODAGT-368
Relates to https://github.com/coder/internal/issues/1397 & Relates to
CODAGT-374

coderd/coderd.go

@@ -95,6 +95,7 @@ import (
 	"github.com/coder/coder/v2/coderd/workspacestats"
 	"github.com/coder/coder/v2/coderd/wsbuilder"
 	"github.com/coder/coder/v2/coderd/x/chatd"
+	"github.com/coder/coder/v2/coderd/x/chatd/chatprovider"
 	"github.com/coder/coder/v2/coderd/x/chatd/mcpclient"
 	"github.com/coder/coder/v2/coderd/x/gitsync"
 	"github.com/coder/coder/v2/codersdk"
@@ -248,6 +249,9 @@ type Options struct {
 	// ChatSubscribeFn provides cross-replica subscription merging.
 	// Set by enterprise for HA deployments. Nil in AGPL single-replica.
 	ChatSubscribeFn chatd.SubscribeFn
+	// ChatProviderAPIKeys overrides deployment-derived provider keys.
+	// Test harnesses use this to route chat models to local providers.
+	ChatProviderAPIKeys *chatprovider.ProviderAPIKeys
 
 	UpdateAgentMetrics func(ctx context.Context, labels prometheusmetrics.AgentMetricLabels, metrics []*agentproto.Stats_Metric)
 	StatsBatcher       workspacestats.Batcher
@@ -792,13 +796,18 @@ func New(options *Options) *API {
 				options.Logger.Named("mcp-user-oidc"),
 			)
 		}
+		providerAPIKeys := ChatProviderAPIKeysFromDeploymentValues(options.DeploymentValues)
+		if options.ChatProviderAPIKeys != nil {
+			providerAPIKeys = *options.ChatProviderAPIKeys
+		}
+
 		api.chatDaemon = chatd.New(chatd.Config{
 			Logger:                         options.Logger.Named("chatd"),
 			Database:                       options.Database,
 			ReplicaID:                      api.ID,
 			SubscribeFn:                    options.ChatSubscribeFn,
 			MaxChatsPerAcquire:             int32(maxChatsPerAcquire), //nolint:gosec // maxChatsPerAcquire is clamped to int32 range above.
-			ProviderAPIKeys:                ChatProviderAPIKeysFromDeploymentValues(options.DeploymentValues),
+			ProviderAPIKeys:                providerAPIKeys,
 			AlwaysEnableDebugLogs:          options.DeploymentValues.AI.Chat.DebugLoggingEnabled.Value(),
 			AgentConn:                      api.agentProvider.AgentConn,
 			AgentInactiveDisconnectTimeout: api.AgentInactiveDisconnectTimeout,