WebRTC Bug Fixes, Diagnostics, and Backpressure

[timwu20]

Stacked on top of paritytech/litep2p#554 (haiko's multistream-select negotiation fix). These changes address several issues discovered during testing with browser clients (smoldot) and improve reliability of the WebRTC transport.

Changes

Bug Fixes:

• Fix InputRejected error during ICE negotiation (594520e) — Route all packets from existing opening connections to those connections instead of creating duplicate Rtc instances. Remove premature accepts() check that was rejecting valid packets before ICE candidate nomination completed.
• Fix FIN-after-handler-drop race (130a647) — When a request-response handler writes a response and drops the Substream, a subsequent FIN from the remote peer no longer causes EssentialTaskClosed. Adds fin_delivered tracking to distinguish clean closes from unclean drops, emitting ResetStream only when appropriate.
• Report substream open failure (d51dd01) — When a data channel closes during DCEP or multistream-select negotiation, report the failure to the protocol layer so it can retry instead of silently losing the substream.
• Close substream before sending feedback (e72cd9d) — In request-response, call substream.close() before sending the feedback oneshot, ensuring the FIN is sent before the caller is notified of success.
• Return error when SCTP write buffer is full (ce6d2ae) — channel.write() returns Ok(false) when the buffer is full; previously the bool was silently discarded, causing data loss. Now returns Error::ChannelClogged.

Backpressure:

• Add network-level backpressure (be4c14b) — When channel.write() rejects data (global 128 KB SCTP buffer full), apply backpressure to the Substream's AsyncWrite impl so writers block instead of losing data. Pending writes are queued and retried.
• Retry pending writes on Output::Transmit (4cefe69) — Replace per-channel ChannelBufferedAmountLow event with retry on Output::Transmit, which correctly handles the mismatch between str0m's global buffer limit and per-channel events.

Diagnostics:

• Add trace logging for notification handshake (99498b8) — Trace-level logging for handshake negotiation flow: payload forwarding, handshake reads, and timeout reporting.

Dependency:

• Update str0m to ChainSafe fork (8a7082f) — Migrate to ChainSafe's str0m fork with updated DTLS cert API and Rtc::builder().build(Instant::now()).

Test plan

• All 328 unit tests pass (cargo test --features webrtc --lib)
• rustfmt (nightly) clean
• clippy clean
• Verify large responses (>128 KB) are no longer truncated with backpressure