fix: lazily project union contents

[agoose77]

This fixes #2911 by changing the projection policy of unions, as described in #2911 (comment)

ping @lgray

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a9c2e3b) 81.98% compared to head (b647a64) 81.98%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/contents/unionarray.py	85.22% <100.00%> (ø)
src/awkward/operations/ak_enforce_type.py	81.97% <100.00%> (ø)

... and 1 file with indirect coverage changes

[lgray]

Excellent, the test case is fixed up:

/Users/lgray/coffea-dev/coffea/src/coffea/nanoevents/schemas/nanoaod.py:243: RuntimeWarning: Missing cross-reference index for FatJet_genJetAK8Idx => GenJetAK8
  warnings.warn(
{'from-uproot-b1b15c47a073739766e8fd9b6aac75c1': frozenset({'Electron_eta', 'Electron_pt', 'Muon_eta', 'Electron_charge', 'Electron_mass', 'Muon_phi', 'Muon_pt', 'nElectron', 'Electron_phi', 'nMuon', 'Muon_mass', 'Muon_charge'})}
[[], [59.1], [120, 103], [21.3, 17.2], [154, ...], ..., [], [15.4], [], []]

as is the full example from the benchmarks: