[Repo Assist] feat: wire WebView2 bidirectional native↔SPA bridge in WebChatWindow

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Implements the Windows side of the WebView2 bridge proposed in issue #191.

What changed

OpenClaw.Shared — WebBridgeMessage

New WebBridgeMessage record defining the bridge message contract:

{ "type": "...", "payload": { ... } }

• Well-known type constants matching issue [Proposal] Add a formal WebView2 bridge for native ↔ SPA communication #191's contract:
  recording-start, recording-stop, voice-start, voice-stop, draft-text, ready
• TryParse(string?) — null-safe JSON parser, returns null on malformed input
• ToJson(object?) — serialiser; runtime payload overrides stored PayloadJson

WebChatWindow.xaml.cs

SPA → native (incoming messages):

// Subscribe in InitializeWebViewAsync, unsubscribed in OnWindowClosed
WebView.CoreWebView2.WebMessageReceived += _webMessageReceivedHandler;
// Callers subscribe to:
public event EventHandler<WebBridgeMessage>? BridgeMessageReceived;

The SPA sends via window.chrome.webview.postMessage({ type, payload }) — window.chrome.webview is automatically available in WebView2-hosted pages.

Native → SPA (outgoing messages):

public void PostBridgeMessage(string type, object? payload = null)

The SPA receives via window.chrome.webview.addEventListener('message', e => { const msg = e.data; ... }).

Handler is stored in _webMessageReceivedHandler and unregistered in OnWindowClosed — no event-handler leak.

Why this matters

From issue #191 — three open PRs need this infrastructure:

PR / Feature	What it unlocks
#120 Voice Mode (NichUK)	Replace fragile DOM scraping with voice-start/voice-stop bridge messages
#159 Screen recording (AlexAlves87)	Show recording indicator in SPA via recording-start/recording-stop
Future	Push draft text to chat input via draft-text bridge message

What's NOT in scope

• Gateway/SPA side: adding window.chrome.webview.addEventListener in the gateway SPA (tracked in [Proposal] Add a formal WebView2 bridge for native ↔ SPA communication #191 as a separate repo concern)
• CanvasWindow: already has ExecuteScriptAsync channel; bridge upgrade can be a separate PR
• NodeService wiring: posting recording-start from ScreenCapability is a follow-on once the SPA side is ready

Test Status

• dotnet test OpenClaw.Shared.Tests — 648 passed, 20 skipped (↑15 from 633; 15 new WebBridgeMessageTests)
• dotnet test OpenClaw.Tray.Tests — 122 passed (unchanged)

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[AlexAlves87]

Thanks for picking up the Windows host side of the bridge from #191.

I opened the matching SPA-side draft PR here: openclaw/openclaw#69633

That PR adds the window.chrome.webview listener/ready-handshake layer on the frontend side, so both halves of the bridge can be reviewed together.