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/components/SandboxedIframe.ts

@@ -169,7 +169,7 @@ export class SandboxIframe extends LitElement {
 		return new Promise((resolve, reject) => {
 			// 4. Create execution consumer for lifecycle messages
 			const executionConsumer: MessageConsumer = {
-				handleMessage(message: any): boolean {
+				async handleMessage(message: any): Promise<boolean> {
 					if (message.type === "console") {
 						logs.push({
 							type: message.method === "error" ? "error" : "log",