[BUG] Fix runner check at plan execution time for new query planner

[clarkzinzow]

This PR fixes is_ray_runner check at plan execution time in the new query planner. Previously, we were checking the Daft context at execution time, which wouldn't be properly set under the Ray runner, since we don't propagate the Daft context to all Ray workers. This PR changes the runner check to be based on an explicit flag passed from the runner to the physical plan execution API.

Closes #1433

[codecov]

Codecov Report

Merging #1435 (8d6067b) into main (ecb596a) will decrease coverage by 0.01%.
The diff coverage is 85.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1435      +/-   ##
==========================================
- Coverage   74.51%   74.51%   -0.01%     
==========================================
  Files          60       60              
  Lines        6035     6034       -1     
==========================================
- Hits         4497     4496       -1     
  Misses       1538     1538              

Files	Coverage Δ
daft/execution/rust_physical_plan_shim.py	98.18% <100.00%> (-0.04%)	⬇️
daft/planner/planner.py	87.50% <100.00%> (ø)
daft/planner/rust_planner.py	100.00% <100.00%> (ø)
daft/runners/pyrunner.py	95.80% <100.00%> (ø)
daft/runners/ray_runner.py	91.20% <100.00%> (ø)
daft/planner/py_planner.py	0.00% <0.00%> (ø)