fix(selfhost): surface failed orb relay drains

[JSONbored]

Motivation

• The pull-mode drain path treated any resolved drain result (including []) as proof of broker progress, but the production drainOrbRelay collapsed non-OK HTTP responses and caught errors into [], making broker failures indistinguishable from successful empty polls.
• This caused the no-progress monitoring gauge/alert to be falsely healthy during broker outages, delaying operator detection of a stuck pull-mode relay.

Description

• Change drainOrbRelay in src/orb/broker-client.ts to throw on non-OK broker responses and propagate thrown/rejected errors instead of returning [], while preserving the [] no-op when not in broker mode.
• Add unit tests in test/unit/orb-broker-client.test.ts to assert the new drainOrbRelay behavior for non-OK responses, thrown errors, and unsafe URLs.
• Add a regression test in test/unit/selfhost-monitored-work.test.ts that wires the real drainOrbRelay through drainOrbRelayWithMonitor and verifies a 503 broker response does not stamp lastDrainAtMs or clear pending acks.

Testing

• Ran npm run typecheck and it completed without type errors.
• Ran the targeted unit tests with npm test -- --run test/unit/orb-broker-client.test.ts test/unit/selfhost-monitored-work.test.ts and all tests passed (55 tests across the two files).
• Attempted a full unsharded coverage run with npm run test:coverage; the full coverage job was observed to take an extended time and was terminated to keep the submit cycle bounded, so a complete unsharded coverage report was not included in this PR submission.

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Codex Task

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.29%. Comparing base (de375d3) to head (a4d5633).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3785   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files         317      317           
  Lines       32368    32369    +1     
  Branches    11872    11873    +1     
=======================================
+ Hits        30199    30200    +1     
  Misses       1537     1537           
  Partials      632      632           

Files with missing lines	Coverage Δ
src/orb/broker-client.ts	99.11% <100.00%> (+<0.01%)	⬆️

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