fix(server): share gateway shutdown channel

[elezar]

Summary

Fix the main CI Rust lint failure from https://github.com/NVIDIA/OpenShell/actions/runs/27656754843/job/81792472668 by removing the duplicate gateway shutdown channel introduced by the merge of #1870 onto a main that already included #1577.

PR #1870 passed checks on stale head 29d57cc, which did not include #1577. The final merge commit ff028ce0 combined #1870 TLS reload shutdown handling with #1577 compute watcher shutdown handling, shadowing shutdown_tx and leaving the first sender unused under -D warnings.

Related Issue

None.

Changes

• Reuse the existing run_server shutdown channel for compute watchers, TLS reload, and gateway listener shutdown.
• Remove the second watch::channel(false) allocation that shadowed shutdown_tx and shutdown_rx.

Testing

• mise run rust:lint passes
• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

mise run pre-commit was attempted locally and failed in the unrelated helm:lint task before this PR changes any Helm files: chart metadata is missing these dependencies: postgresql. Rust lint, rust check, and Rust tests completed successfully during that run before the overall task exited non-zero.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)