fix(client): preserve reasoning messages after MESSAGES_SNAPSHOT (#1370)

Reasoning messages (role: 'reasoning') are emitted during streaming
via REASONING_MESSAGE_START/CONTENT events but are not included in
the final MESSAGES_SNAPSHOT (reasoning is metadata on AI chunks, not
a LangChain message type).

The snapshot filter in default.ts only exempts 'activity' role from
the snapshot-based filter, so reasoning messages are dropped when the
snapshot arrives — they disappear from agent.messages after the run.

Fix: Extract a helper isClientOnlyRole() that exempts both 'activity'
and 'reasoning' roles from snapshot filtering, preserving reasoning
messages across the snapshot merge.

Closes #1262

Co-authored-by: giulio-leone <giulio.leone@users.noreply.github.com>
Co-authored-by: Alem Tuzlak <t.zlak97@gmail.com>

Author: 3 people

sdks/typescript/packages/client/src/apply/default.ts

@@ -540,14 +540,18 @@ export const defaultApplyEvents = (
             const { messages: newMessages } = event as MessagesSnapshotEvent;
 
             // Edit-based merge: update existing messages with snapshot data while
-            // preserving activity messages (which the backend doesn't know about).
+            // preserving activity and reasoning messages (which the backend
+            // doesn't include in the snapshot).
             const snapshotMap = new Map(newMessages.map((m) => [m.id, m]));
 
-            // Step 1 + 2: Keep activity messages as-is, keep messages present in
-            // the snapshot (replaced with snapshot version), drop everything else.
+            // Step 1 + 2: Keep activity/reasoning messages as-is, keep messages
+            // present in the snapshot (replaced with snapshot version), drop
+            // everything else.
+            const isClientOnlyRole = (role: string) =>
+              role === "activity" || role === "reasoning";
             messages = messages
-              .filter((m) => m.role === "activity" || snapshotMap.has(m.id))
-              .map((m) => (m.role === "activity" ? m : snapshotMap.get(m.id)!));
+              .filter((m) => isClientOnlyRole(m.role) || snapshotMap.has(m.id))
+              .map((m) => (isClientOnlyRole(m.role) ? m : snapshotMap.get(m.id)!));
 
             // Step 3: Append messages from the snapshot that we don't have yet.
             const existingIds = new Set(messages.map((m) => m.id));