fix(sdk,chat): make isStreaming optional in reconnectToStream short-circuit

TriggerChatTransport.reconnectToStream previously returned null any time
state.isStreaming was falsy, which included undefined. That meant a
caller who dropped isStreaming from their ChatSession persistence (a
reasonable simplification now that the server can tell the client when
a session is settled via X-Session-Settled on the session.out SSE)
would get null on every reconnect and the UI would never resume
streaming.

Tighten the check to state.isStreaming === false so only an explicit
false triggers the fast-path skip. Undefined now falls through to open
the SSE and let the server decide — on a settled session the server
already closes the connection in ~1s via wait=0, so there is no 60s
hang to worry about.

Backward compatible: callers who still persist and hydrate isStreaming
(true/false) keep today's behavior exactly; callers who drop the flag
now get the server-authoritative path.

Author: ericallam

.changeset/chat-reconnect-isstreaming-optional.md

@@ -0,0 +1,5 @@
+---
+"@trigger.dev/sdk": patch
+---
+
+`TriggerChatTransport.reconnectToStream` no longer requires callers to persist an `isStreaming` flag in `ChatSession` state. Previously, any falsy `isStreaming` (including `undefined` when the flag was dropped from persistence) short-circuited reconnect to `null` and left the UI hanging on incomplete streams. Now the short-circuit only triggers on an explicit `isStreaming === false`, so callers can drop the flag entirely and let the server decide via the session's own `.out` tail. Existing callers that still persist `isStreaming` are unaffected.

packages/trigger-sdk/src/v3/chat.ts

@@ -827,9 +827,13 @@ export class TriggerChatTransport implements ChatTransport<UIMessage> {
     const state = this.sessions.get(options.chatId);
     if (!state) return null;
 
-    // No active stream — the last turn completed before the page refreshed.
-    // Return null so useChat settles into "ready" state instead of hanging.
-    if (!state.isStreaming) return null;
+    // Fast-path: if the caller still tracks `isStreaming` in their
+    // persisted state and knows the last turn completed, skip the SSE
+    // open. Undefined (caller dropped the flag) falls through and the
+    // server decides via the peek-tail-settled path — on a settled
+    // session the SSE uses wait=0 and closes immediately, so there's
+    // no 60s hang to worry about.
+    if (state.isStreaming === false) return null;
 
     // Deduplicate: if there's already an active stream for this chatId,
     // return null so the second caller no-ops.