fix(tts): don't fail the session while FallbackAdapter is falling back

.changeset/tts-fallback-child-errors.md

@@ -0,0 +1,5 @@
+---
+'@livekit/agents': patch
+---
+
+Stop `tts.FallbackAdapter` from closing the session while it is falling back. Child `error` events were re-emitted verbatim, so one provider failure (an ElevenLabs 401, say) closed the session even though the next instance was already serving audio, and the recovery probe re-raised it every `recoveryDelayMs`. The adapter now absorbs a child's failure when it recovers from it, logging the provider's own error, and reports its own error when it can't: every instance failed, or speech was already cut off. A recovery probe that errors no longer counts as recovered, `StreamAdapter` wrappers no longer stay subscribed to non-streaming instances, `close()` removes only the adapter's own listeners, and a failed `ChunkedStream` no longer surfaces as an unhandled rejection.

agents/etc/agents.api.md

@@ -2657,6 +2657,8 @@ abstract class ChunkedStream implements AsyncIterableIterator<SynthesizedAudio>
     // (undocumented)
     protected closed: boolean;
     collect(): Promise<AudioFrame>;
+    // @internal
+    get error(): Error | undefined;
     // (undocumented)
     get inputText(): string;
     // (undocumented)
@@ -7917,6 +7919,8 @@ class StreamAdapterWrapper extends SpeechStream {
 class StreamAdapterWrapper_2 extends SynthesizeStream {
     constructor(tts: TTS, sentenceTokenizer: SentenceTokenizer, connOptions?: APIConnectOptions);
     // @internal
+    get error(): Error | undefined;
+    // @internal
     get expressive(): boolean;
     // (undocumented)
     label: string;
@@ -8208,6 +8212,8 @@ abstract class SynthesizeStream implements AsyncIterableIterator<SynthesizedAudi
     // (undocumented)
     static readonly END_OF_STREAM: unique symbol;
     endInput(): void;
+    // @internal
+    get error(): Error | undefined;
     flush(): void;
     // (undocumented)
     protected static readonly FLUSH_SENTINEL: unique symbol;