fix(agent-to-agent): route A2A replies back to originating session (#2267)

Squash merge of PR #2267 by ddaniels.

When an agent group has more than one active session, A2A replies landed
in the newest session via findSessionByAgentGroup's ORDER BY created_at
DESC. The session that asked the question never saw the answer.

Adds origin-aware return-path routing with three layers:

1. Direct return-path: if the reply has in_reply_to, look up the
   triggering inbound row's source_session_id and route there.
2. Peer-affinity fallback: find the most recent A2A inbound from this
   peer and use its source_session_id.
3. Legacy fallback: newest active session (pre-migration compat).

Container-side: MCP send_message/send_file now thread the current
batch's in_reply_to through to outbound rows via current-batch.ts.

Also flips our A2A bug-documenting test (#2332) from asserting the
broken behavior to asserting the fixed behavior.

Co-Authored-By: Doug Daniels <ddaniels888@gmail.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/current-batch.ts

@@ -0,0 +1,29 @@
+/**
+ * Per-batch context the poll loop publishes for downstream consumers
+ * (MCP tools, etc.) that don't sit on the poll-loop's call stack.
+ *
+ * Today the only field is `inReplyTo` — the id of the first inbound
+ * message in the batch the agent is currently processing. MCP tools like
+ * `send_message` and `send_file` read this and stamp it onto the outbound
+ * row so the host's a2a return-path routing can correlate replies back to
+ * the originating session.
+ *
+ * This is module-level state on purpose: the agent-runner is single-process
+ * and processes one batch at a time. Poll-loop calls `setCurrentInReplyTo`
+ * before invoking the provider and `clearCurrentInReplyTo` after the batch
+ * completes (or errors out).
+ */
+let currentInReplyTo: string | null = null;
+
+export function setCurrentInReplyTo(id: string | null): void {
+  currentInReplyTo = id;
+}
+
+export function clearCurrentInReplyTo(): void {
+  currentInReplyTo = null;
+}
+
+export function getCurrentInReplyTo(): string | null {
+  return currentInReplyTo;
+}
+