Cherry-pick from upstream

• Upstream commit: df65ed7e9
• Author: Peter Steinberger [email protected]
• Tier: AUTO-PICK
• Issue: Cherry-pick: Agent Delivery & Session Context #658 (Batch 5)
• Depends on: Cherry-pick 712e23172: fix(agent): forward resolved outbound session context for delivery #1216

Summary

Aligns outbound session assertion shapes in gateway send and message tests. Tests now assert against the session object (with agentId + key) instead of the flat agentId property, matching the session context forwarding changes from the previous commit.

Conflict Resolution

• send.test.ts: 4 identical conflicts — fork (Batch 4) already wrapped agentId in session object but without key. Took upstream's more specific assertions that include both agentId and key.
• message.test.ts: Both sides converged on session wrapper; fork changed channel from telegram to mattermost (rebrand). Kept fork's channel/to values with the session wrapper.

Cherry-picked-from: df65ed7