Fix infinite loop when ErrStreamAlreadyExist occurs for in-band data channels

[timwu20]

Summary

• Fix three error paths in RtcSctp::do_poll() where continue skips close handling, causing entries to stay in AwaitOpen state indefinitely
• Affected entries are now immediately transitioned to Closed and emit SctpEvent::Close

Problem

When an in-band (DCEP) data channel fails to open with ErrStreamAlreadyExist, the stream entry has do_close = true set but the continue statement skips the close handling at the bottom of the loop. Since state remains AwaitOpen, subsequent polls retry the open — creating an infinite loop of warnings and eventually causing a panic (writing dcep open: ErrPayloadDataStateNotExist).

This is triggered when both peers try to open data channels simultaneously and the automatic stream ID allocation conflicts with a stream already opened by the remote peer.

Fix

In do_poll(), three error paths inside the if want_open block used continue after setting do_close = true, which skipped the close-handling code further down the loop body:

1. configure_reliability failure
2. ErrStreamAlreadyExist for in-band channels
3. Generic open_stream errors

All three now transition the entry to Closed and return SctpEvent::Close directly instead of continue-ing.

Test plan

• Existing test suite passes (cargo test — 0 failures)
• Manual verification with simultaneous data channel opens from both peers