fix: propagate threaded provider errors

Summary

• Preserve provider exception types and messages when run_sync() is called from an active asyncio loop.
• Replace the manual thread result dictionary with a single-worker future, whose result() re-raises worker exceptions in the caller.
• Add a deterministic regression for the previous RuntimeError → KeyError("value") failure.

Fixes #503.

Regression evidence

• Before: uv run python -m pytest tests/legacy/test_client.py -q exited 1 with KeyError: 'value'.
• After: the same command exited 0 with 5 passed.

Verification

• make ci
  • root tests: 134 passed
  • maintained-agent tests: 118 passed, 1 skipped
  • Ruff lint and format checks passed
  • Mypy passed
  • root and maintained-agent package builds passed

Scope

• 2 files changed, +13 / -11 lines
• no dependency, lockfile, generated-file, or unrelated changes