fix(bridge): emit empty content delta as heartbeat; preserve user provider fields on reinstall

OpenClaw's LLM idle watchdog (default 120s) fires on lack of *model
progress*, not lack of bytes — an SSE comment frame (": keepalive\n\n")
keeps the TCP socket alive but isn't recognized as progress, so a long
quiet tool-call phase still idles out. When that happens OpenClaw falls
back to re-sending the prior turn's assistant text (pi-embedded:1308
fallbackAnswerText), producing duplicate-Discord-message symptoms.

Heartbeat now emits a real chat.completion.chunk with an empty content
delta every 30s. Clients drop empty deltas; the upstream idle watchdog
should count it as model progress because it's a real event on the
canonical streaming channel.

scripts/install.mjs now spreads the existing provider entry before
overriding script-managed fields, so user-added fields like
timeoutSeconds survive reinstall.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/install.mjs

@@ -67,10 +67,15 @@ function install() {
   // 3. Update openclaw.json
   const cfg = readConfig();
 
-  // Add provider
+  // Add provider — spread existing first so user-added fields
+  // (e.g. timeoutSeconds, extraHeaders) survive reinstall. Script-managed
+  // fields (baseUrl/apiKey/api/models) are then overridden authoritatively
+  // since they're tied to the constants and model catalog above.
   cfg.models = cfg.models ?? {};
   cfg.models.providers = cfg.models.providers ?? {};
+  const existingProvider = cfg.models.providers[PLUGIN_ID] ?? {};
   cfg.models.providers[PLUGIN_ID] = {
+    ...existingProvider,
     baseUrl: `http://127.0.0.1:${BRIDGE_PORT}/v1`,
     apiKey: BRIDGE_API_KEY,
     api: "openai-completions",