Fix empty SDP in pooled service offers

The publishInitialService() method was creating an offer with SDP
but not returning it. This caused the first offer in the pool to
have an empty SDP string, which failed when trying to set it as
the local description when an answer arrived.

Fixed by:
- Storing the offer SDP before closing the peer connection
- Adding offerSdp to the return value of publishInitialService()
- Using the returned SDP when creating the initial offer in the pool

This ensures all offers in the pool have valid SDP that can be
used to recreate the peer connection state when answers arrive.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/service-pool.ts

@@ -143,9 +143,9 @@ export class ServicePool {
       onError: (err, ctx) => this.handleError(err, ctx)
     });
 
-    // Add all offers to pool
+    // Add all offers to pool (include the SDP from the initial offer)
     const allOffers = [
-      { id: service.offerId, peerId: this.credentials.peerId, sdp: '', topics: [], expiresAt: service.expiresAt, lastSeen: Date.now() },
+      { id: service.offerId, peerId: this.credentials.peerId, sdp: service.offerSdp, topics: [], expiresAt: service.expiresAt, lastSeen: Date.now() },
       ...additionalOffers
     ];
     await this.offerPool.addOffers(allOffers);
@@ -382,6 +382,7 @@ export class ServicePool {
     serviceId: string;
     uuid: string;
     offerId: string;
+    offerSdp: string;
     expiresAt: number;
   }> {
     const { username, privateKey, serviceFqn, rtcConfig, isPublic, metadata, ttl } = this.options;
@@ -402,6 +403,9 @@ export class ServicePool {
       throw new Error('Failed to generate SDP');
     }
 
+    // Store the SDP before closing
+    const offerSdp = offer.sdp;
+
     // Create signature
     const timestamp = Date.now();
     const message = `publish:${username}:${serviceFqn}:${timestamp}`;
@@ -417,7 +421,7 @@ export class ServicePool {
       body: JSON.stringify({
         username,
         serviceFqn,
-        sdp: offer.sdp,
+        sdp: offerSdp,
         ttl,
         isPublic,
         metadata,
@@ -439,6 +443,7 @@ export class ServicePool {
       serviceId: data.serviceId,
       uuid: data.uuid,
       offerId: data.offerId,
+      offerSdp,
       expiresAt: data.expiresAt
     };
   }