relay_bridge: await the pump task instead of sleeping 50 ms

[jeff-hykin]

Fix flakey test

[greptile-apps]

Greptile Summary

This change replaces fixed sleeps in relay pump lifecycle tests with bounded awaits of the pump task. The encode-error and clean session-close paths completed as intended in focused runs, ten repeated scheduling runs, and the complete relay WebTransport client test module.

Confidence Score: 5/5

Safe to merge: the updated lifecycle assertions directly observe the pump task's terminal outcome and passed all exercised paths.

Focused testing confirmed that encoding failures propagate as ValueError, session closure completes normally, and later offers are rejected in both cases. Repeated runs showed no timeout, hang, or scheduling flake.

Files Needing Attention: No files need further attention.

T-Rex Logs

What T-Rex did

• Restored the CPython 3.12 interpreter in the prepared virtual environment and ran the targeted pytest tests pump_dies_visibly_on_encode_error and pump_stops_cleanly_on_session_close; both tests passed with exit code 0. The run also captured a repository-noted non-fatal ResourceWarning that did not affect the results.
• Ran the two lifecycle tests ten consecutive times and all executions completed without timeouts, hangs, or scheduling failures.
• Executed the complete dimos/web/relay_bridge/test_wt_client.py module and all 21 tests passed.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "relay_bridge: await the pump task instea..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

@@            Coverage Diff             @@
##             main    #3877      +/-   ##
==========================================
- Coverage   77.64%   77.64%   -0.01%     
==========================================
  Files        1323     1323              
  Lines      125419   125416       -3     
  Branches    10911    10911              
==========================================
- Hits        97384    97377       -7     
- Misses      24890    24892       +2     
- Partials     3145     3147       +2     

Flag	Coverage Δ
OS-ubuntu-24.04-arm	74.31% <100.00%> (-0.01%)	⬇️
OS-ubuntu-latest	74.85% <100.00%> (-0.01%)	⬇️
Py-3.10	74.85% <100.00%> (+<0.01%)	⬆️
Py-3.11	74.85% <100.00%> (-0.01%)	⬇️
Py-3.12	74.85% <100.00%> (-0.01%)	⬇️
Py-3.13	74.85% <100.00%> (-0.01%)	⬇️
Py-3.14	74.85% <100.00%> (-0.01%)	⬇️
Py-3.14t	74.85% <100.00%> (-0.01%)	⬇️
SelfHosted-Large	30.21% <0.00%> (-0.01%)	⬇️
SelfHosted-Linux	34.72% <0.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
dimos/web/relay_bridge/test_wt_client.py	98.15% <100.00%> (-0.03%)	⬇️

... and 3 files with indirect coverage changes

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