fix(asr): skip spurious empty Interim events in qwen and doubao providers

[erning]

Summary

• Replace AsrEvent::Interim(String::new()) fallbacks with a loop that skips meaningless WS frames (binary, ping/pong, empty parse results) and waits for the next real event
• Each skipped frame logs at debug level for protocol debugging
• Close, error, and stream-end branches remain unchanged

Problem

Both QwenProvider::next_event() and DoubaoProvider::next_event() return a fabricated empty Interim("") event when they encounter non-data frames (binary, ping/pong) or when parse_server_event produces no events. Providers should not synthesize transcript events for frames that carry no speech data. While the core layer already guards against empty text (!text.is_empty()), the spurious events still cause unnecessary polling and pollute debug logs.

Fix

Wrap the WS read in a loop so meaningless frames are skipped via continue instead of producing fake events. This cleanly separates "ignore this frame" from "emit a real ASR event" without touching the terminal branches (Close / Err / None).

Files Changed

• koe-asr/src/qwen.rs — next_event()
• koe-asr/src/doubao.rs — next_event()

Test Plan

• cargo check -p koe-asr passes
• cargo test -p koe-asr — all 10 tests pass
• Verify hold-to-talk and tap-to-toggle both work with Qwen provider
• Verify hold-to-talk and tap-to-toggle both work with Doubao provider