Add artifact message persistence for session reconstruction

- Add ArtifactMessage type as core part of AppMessage union (not CustomMessages)
- ArtifactsRuntimeProvider appends artifact messages on create/update/delete
- MessageList filters out artifact messages (UI display only)
- artifacts.ts reconstructFromMessages handles artifact messages
- Export ARTIFACTS_RUNTIME_PROVIDER_DESCRIPTION from main index
- Fix artifact creation bug: pass filename as title instead of mimeType

Changes:
- web-ui/src/components/Messages.ts: Add ArtifactMessage to BaseMessage union
- web-ui/src/components/MessageList.ts: Skip artifact messages in render
- web-ui/src/components/sandbox/ArtifactsRuntimeProvider.ts: Append messages, fix title parameter
- web-ui/src/ChatPanel.ts: Pass agent.appendMessage callback
- web-ui/src/tools/artifacts/artifacts.ts: Handle artifact messages in reconstructFromMessages
- web-ui/src/index.ts: Export ARTIFACTS_RUNTIME_PROVIDER_DESCRIPTION
- web-ui/example/src/custom-messages.ts: Update message transformer to filter artifacts

packages/web-ui/src/tools/artifacts/HtmlArtifact.ts

@@ -16,7 +16,6 @@ import "./Console.js";
 @customElement("html-artifact")
 export class HtmlArtifact extends ArtifactElement {
 	@property() override filename = "";
-	@property({ attribute: false }) override displayTitle = "";
 	@property({ attribute: false }) runtimeProviders: SandboxRuntimeProvider[] = [];
 	@property({ attribute: false }) sandboxUrlProvider?: () => string;
 
@@ -87,7 +86,7 @@ export class HtmlArtifact extends ArtifactElement {
 
 		// Create consumer for console messages
 		const consumer: MessageConsumer = {
-			handleMessage: (message: any): boolean => {
+			handleMessage: async (message: any): Promise<boolean> => {
 				if (message.type === "console") {
 					// Create new array reference for Lit reactivity
 					this.logs = [