test(gfql): clarify polars-gpu contracts - #2010

Summary

• add 20 explicit engine="polars-gpu" strictness cells for absent-label/property values and strict/warn/quiet behavior
• classify only an observed Polars-GPU fused-lane capability decline as an expected xfail, after fallback values match eager/pandas oracles
• keep CPU Polars and any future serving GPU path as strict must-serve assertions

Stack / landing

• predecessor fix(gfql): preflight remote engine requests #2005 landed on master as merge commit 749d590894e2f2a1192b4ddd1f098351d8d11832
• rebased directly onto that master; exact head 9a8934d7f40da4cc8ed39d9b5330b39c051d6f1a
• admin self-merge is authorized only after replacement CI is terminal green and no new actionable review appears

Scope

Test-only behavior plus one Development changelog entry: 3 files, +93/-14 across two atomic commits. No product code.

Validation

• local focused pytest on the byte-identical pre-rebase tree: 398 passed, 213 expected unavailable-GPU skips, 0 failed
• Ruff: passed on both touched tests
• changed-test mypy with --follow-imports=skip: 0 issues
• diff-check and prohibited added-line scan: clean
• rebase range-diff: both commits patch-identical; old/new head trees are byte-identical at 26b331ab8536548ac9d1c1a8bc220467c5ac6577
• structured self-review converged after four waves; its sole important test-completeness finding is fixed, with zero unresolved findings
• replacement hosted CI is running on exact head 9a8934d7f40da4cc8ed39d9b5330b39c051d6f1a
• shared DGX validation remains deferred because repeated preflights found an active Python GPU process; no workload or container was launched

Closes #1997