fix: remove erroneous async keyword from sync ActorClient.default_build

[vdusek]

Summary

• ActorClient.default_build (the sync variant) was incorrectly declared as async def — a copy-paste error from the async variant
• Calling it returned a coroutine object instead of a BuildClient, making it unusable without await (which makes no sense on the sync client)
• Removed the async keyword so it behaves as a regular synchronous method

Test plan

• Verify existing unit tests pass
• Verify ActorClient(...).default_build() returns a BuildClient directly (not a coroutine)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.02%. Comparing base (e7ee8b3) to head (e23c75f).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #620   +/-   ##
=======================================
  Coverage   76.02%   76.02%           
=======================================
  Files          42       42           
  Lines        2482     2482           
=======================================
  Hits         1887     1887           
  Misses        595      595           

Flag	Coverage Δ
integration	68.77% <100.00%> (ø)
unit	64.66% <100.00%> (ø)

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