fix: add onInboundStreamNotification to keep onStreamAvailable backwards compatible

[smoghe-bw]

Summary

• The gateway SSRC mapping change causes a streamAvailable WS notification to fire before the WebRTC ontrack event arrives. The feature branch wired this notification through onStreamAvailable, but that callback previously always carried a mediaStream — breaking existing consumers.
• Introduced onInboundStreamNotification as the dedicated callback for the pre-media signaling notification (carries callId/autoAccepted, mediaStream is always undefined here).
• Restored onStreamAvailable to only fire from the WebRTC ontrack handler, where mediaStream is always present. Existing consumers are unaffected.

Before / After

Event	Before (broken)	After (fixed)
Gateway streamAvailable WS	fires onStreamAvailable with no mediaStream	fires onInboundStreamNotification with callId/autoAccepted
WebRTC ontrack	fires onStreamAvailable with mediaStream	fires onStreamAvailable with mediaStream (unchanged)

Migration for consumers using the accept/decline flow

Replace onStreamAvailable usage that inspects callId for accept/decline with onInboundStreamNotification:

// Before (broken with new gateway)
rtc.onStreamAvailable((stream) => {
  if (!stream.mediaStream) showAcceptUI(stream.callId);
});

// After
rtc.onInboundStreamNotification((stream) => {
  showAcceptUI(stream.callId);          // mediaStream is undefined here by design
});
rtc.onStreamAvailable((stream) => {
  playAudio(stream.mediaStream);        // always populated
});

Test plan

• Inbound call with new gateway (SSRC mapping) — onInboundStreamNotification fires first (no media), then onStreamAvailable fires with a populated mediaStream
• Inbound call with old gateway — only onStreamAvailable fires (from ontrack), onInboundStreamNotification never fires
• Existing sample app with no onInboundStreamNotification registered — call connects normally, no errors

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[bwappsec]

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[smoghe-bw]

Superseded — changes pushed directly to metadata-for-stream-events.