fix(sdk): keep SSE reconnect enabled with custom fetch (#2605)

## Summary
- Keep SSE auto-reconnect and idle-reconnect enabled when a custom
`fetch` is supplied (auth/proxy shims), instead of silently forcing
`maxReconnectAttempts: 0`.
- Plumb `maxReconnectAttempts`, `streamIdleReconnect`,
`reconnectDelayMs`, and `onReconnect` through `StreamController` and
React/Vue/Svelte/Angular `useStream`.
- Add transport unit coverage plus browser interrupt tests that drop
`/stream/events` mid-HITL and assert `respond()` still completes.