fix(bridge): dispose superseded session handle on resume/create collision

[jaesbit]

Why

InProcessAgentBridge keys live sessions by sessionId in this.active. A resume or create call for a sessionId that's already tracked there silently overwrote that map entry — the previous handle became orphaned: its subscriptions and any in-flight turn kept running forever with no way to reach or abort it, since getSession only ever returns the newest entry for a given id.

What changed

Before inserting the new handle into this.active, dispose the superseded instance (if any) first, so at most one in-process instance ever drives a given session file at a time.

Testing

• tsc --noEmit clean for apps/server.
• No behavioral test added — this guards an edge case (double resume/create race for the same sessionId) that isn't currently covered by the existing bridge test harness; flagging for follow-up if a repro becomes easy to script against InProcessAgentBridge directly.