fix(transfer): flush partial bridge PCM frame on clean WebSocket close

[dengxiayehu]

Problem

In SipSession::connect_bridge, the bridge forward loop reads PCM16 audio
from the bridge WebSocket and repackages it into fixed 20 ms frames before
forwarding to its sink:

• Pcm sink: samples pushed to a PCM channel
• Track sink: samples (re)sampler → codec encoder → RTC track as RTP frames

The loop only emitted a frame when the buffer held at least
samples_per_frame samples. When the remote side closed the WebSocket
cleanly (Message::Close or stream EOF), any buffered samples shorter than
one frame — up to 20 ms of audio — were silently discarded, because no code
path ever drained a partial buffer.

So the tail of bridge audio was truncated at the end of every cleanly closed
bridge stream. For example, an audio service that streams a prompt over the
bridge and then closes the socket loses the final few milliseconds of the
prompt on the SIP leg.

Fix

• Extract the forward loop into a standalone generic
  bridge_forward_loop<S: Stream<Item = Result<Message, Error>>>. This is a
  behavior-preserving move: it makes the close→flush wiring testable at its
  real boundary with an in-memory stream instead of a live WebSocket.
• Add take_bridge_pcm_frame(buffered, samples_per_frame, flush_tail) as the
  single frame-extraction point, called before sink dispatch. When the stream
  ends cleanly and a non-empty partial frame remains, it zero-pads the tail
  to exactly one full frame and emits it once. Since frame extraction is now
  shared, both sinks benefit: the Pcm sink receives the padded tail frame,
  and the Track path encodes it into a final RTP frame.
• A clean close (Message::Close / EOF) is the only signal that a short
  chunk is final. Deliberate boundaries, unchanged from before:
  • a WebSocket read error still terminates the loop without flushing,
    since the buffered data may be incomplete/corrupt;
  • cancellation via forward_cancel still discards the buffer.

Tests

• test_bridge_forward_flushes_pcm_tail_on_remote_close: feeds 40 samples
  (5 ms at 8 kHz — short of one 20 ms frame) followed by Message::Close
  into bridge_forward_loop with a Pcm sink. Asserts the loop terminates
  on remote close and flushes exactly one 160-sample frame containing the
  original samples followed by zero padding.
• Renamed/extended test_voip_bridge_echo_integration →
  test_voip_bridge_flushes_partial_pcm_tail: sends 25 ms (200 samples)
  through a real echo WebSocket server so that one full frame plus a 5 ms
  tail remains at clean close, then drives the flushed frames through the
  real ChannelAudioSource used by the media bridge egress path and asserts
  the full frame, the padded tail, and subsequent EOF.