fix(sessions): create transcript file on chat.inject when missing (#36645)

`chat.inject` called `appendAssistantTranscriptMessage` with
`createIfMissing: false`, causing a hard error when the transcript
file did not exist on disk despite having a valid `transcriptPath`
in session metadata. This commonly happens with ACP oneshot/run
sessions where the session entry is created but the transcript file
is not yet materialized.

The fix is a one-character change: `createIfMissing: true`. The
`ensureTranscriptFile` helper already handles directory creation
and file initialization safely.

Fixes #36170

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/gateway/server-methods/chat.ts

@@ -1471,7 +1471,7 @@ export const chatHandlers: GatewayRequestHandlers = {
       storePath,
       sessionFile: entry?.sessionFile,
       agentId: resolveSessionAgentId({ sessionKey: rawSessionKey, config: cfg }),
-      createIfMissing: false,
+      createIfMissing: true,
     });
     if (!appended.ok || !appended.messageId || !appended.message) {
       respond(