fix(streams): bound orphaned admin connections

[DavidBabinec]

What changed

• make plugin event-stream teardown idempotent across request aborts, response-consumer cancellation, and failed writes
• remove plugin broadcaster subscribers and heartbeat timers on every disconnect path
• give both the MCP editor bridge and plugin event stream a two-minute server-side lease so orphaned intermediary connections cannot live indefinitely
• rely on the existing client reconnect loops to restore deliberately leased streams
• add server regressions for direct response cancellation and 100 repeated plugin stream connect/cancel cycles

Why

Bun can report a disconnected response consumer through ReadableStream.cancel() without aborting the server Request, and an intermediary may fail to propagate the downstream disconnect at all. Without a bounded lifetime, repeated admin contexts can accumulate orphaned long-lived streams and eventually exhaust available proxy connections. Idempotent teardown handles observable disconnects immediately; the lease bounds the remaining intermediary-orphan case.

Verification

• bun test — 6,229 pass, 0 fail
• focused MCP editor bridge and plugin event-stream tests — 7 pass, 0 fail
• bun run lint
• bunx tsc --noEmit
• bun run build