Y26-066 - [PR] [BUG] Handle top-level exceptions in unified_warehouse / warren

[yoldas]

Closes #

Changes proposed in this pull request

• Added channel reopen support in exchange and subscription layers.
• Wired a shared channel_factory through Den so dependencies can reopen consistently.
• Hardened Fox recovery for closed-channel failures.
• Added pending dead-letter tracking so dead-letter intent survives channel closure.
• Added client-side guard for a running fox with missing consumer to re-enter recovery flow.
• Added/updated specs to cover reopen and recovery scenarios end-to-end.

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version