fix: survive mumble disconnects without restarting audio stream

Guard stream_audio with _is_audio_ready() so that PCM frames are
dropped (not crashed on) when pymumble recreates SoundOutput with
encoder=None during reconnect. The ffmpeg pipeline stays alive,
position tracking remains accurate, and audio feeding resumes once
the codec is negotiated. Listeners hear brief silence instead of
a 30+ second restart with URL re-resolution.

Also adds chat messages to _auto_resume so users see what the bot
intends ("Resuming 'X' at M:SS in a moment" / "...aborted").

tests/test_mumble.py

@@ -2,7 +2,7 @@
 
 import asyncio
 import struct
-from unittest.mock import patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 from derp.mumble import (
     MumbleBot,
@@ -645,3 +645,95 @@ class TestPcmRamping:
         assert samples[1] == 7500
         assert samples[2] == 5000
         assert samples[3] == 2500
+
+
+# ---------------------------------------------------------------------------
+# TestIsAudioReady
+# ---------------------------------------------------------------------------
+
+
+class TestIsAudioReady:
+    def test_no_mumble_object(self):
+        bot = _make_bot()
+        bot._mumble = None
+        assert bot._is_audio_ready() is False
+
+    def test_no_sound_output(self):
+        bot = _make_bot()
+        bot._mumble = MagicMock()
+        bot._mumble.sound_output = None
+        assert bot._is_audio_ready() is False
+
+    def test_no_encoder(self):
+        bot = _make_bot()
+        bot._mumble = MagicMock()
+        bot._mumble.sound_output.encoder = None
+        assert bot._is_audio_ready() is False
+
+    def test_ready(self):
+        bot = _make_bot()
+        bot._mumble = MagicMock()
+        bot._mumble.sound_output.encoder = MagicMock()
+        assert bot._is_audio_ready() is True
+
+    def test_attribute_error_handled(self):
+        bot = _make_bot()
+        bot._mumble = MagicMock()
+        del bot._mumble.sound_output
+        assert bot._is_audio_ready() is False
+
+
+# ---------------------------------------------------------------------------
+# TestStreamAudioDisconnect
+# ---------------------------------------------------------------------------
+
+
+class TestStreamAudioDisconnect:
+    def test_stream_survives_disconnect(self):
+        """stream_audio keeps ffmpeg alive when connection drops mid-stream."""
+        bot = _make_bot()
+        bot._mumble = MagicMock()
+        bot._mumble.sound_output.encoder = MagicMock()
+        bot._mumble.sound_output.get_buffer_size.return_value = 0.0
+
+        frame = b"\x00" * 1920
+        # Track which frame we're on; disconnect after frame 3
+        frame_count = [0]
+        connected = [True]
+
+        async def _fake_read(n):
+            if frame_count[0] < 5:
+                frame_count[0] += 1
+                # Disconnect after 3 frames are read
+                if frame_count[0] > 3:
+                    connected[0] = False
+                return frame
+            return b""
+
+        def _ready():
+            return connected[0]
+
+        proc = MagicMock()
+        proc.stdout.read = _fake_read
+        proc.stderr.read = AsyncMock(return_value=b"")
+        proc.wait = AsyncMock(return_value=0)
+        proc.kill = MagicMock()
+
+        progress = [0]
+
+        async def _run():
+            with patch.object(bot, "_is_audio_ready", side_effect=_ready):
+                with patch("asyncio.create_subprocess_exec",
+                           return_value=proc):
+                    await bot.stream_audio(
+                        "http://example.com/audio",
+                        volume=0.5,
+                        progress=progress,
+                    )
+
+        asyncio.run(_run())
+
+        # All 5 frames were read (progress tracks all, connected or not)
+        assert progress[0] == 5
+        # Only 3 frames were fed to sound_output (the connected ones)
+        assert bot._mumble.sound_output.add_sound.call_count == 3