[#15682] fix: elaborate the do continuation only once when contracting pure ()

[downstream-lean4]

This is the adaptation PR for leanprover/lean4#15682.

[downstream-lean4]

Build report for downstream: follow upstream PR

Stayed green

Repo	Critical	Build	Test	Lint
aesop	✅	✅ in 16s	✅ in 4s	⏭️
batteries	✅	✅ in 12s	✅ in 4s	✅ in 2s
import-graph	✅	✅ in 5s	✅ in 4s	⏭️
lean4-cli	✅	✅ in 3s	✅ in 0s	⏭️
mathlib4	✅	✅ in 15m 26s	✅ in 38s	✅ in 1m 32s
plausible	✅	✅ in 3s	✅ in 2s	⏭️
ProofWidgets4	✅	✅ in 5s	✅ in 1s	⏭️
quote4	✅	✅ in 6s	✅ in 1s	⏭️
reference-manual	✅	✅ in 1m 8s	⏭️	⏭️
BibtexQuery		✅ in 3s	⏭️	⏭️
cslib		✅ in 30s	✅ in 12s	✅ in 3s
doc-gen4		✅ in 14s	⏭️	⏭️
illuminate		✅ in 8s	✅ in 10s	⏭️
lean4-unicode-basic		✅ in 4s	⏭️	⏭️
lean4export		✅ in 3s	✅ in 7s	⏭️
LeanSearchClient		✅ in 2s	✅ in 0s	⏭️
leansqlite		✅ in 9s	✅ in 20s	⏭️
nerodia		✅ in 6s	✅ in 22s	⏭️
repl		✅ in 4s	✅ in 2m 23s	⏭️
verso		✅ in 1m 38s	✅ in 3m 19s	⏭️
verso-slides		✅ in 20s	✅ in 9s	⏭️
verso-web-components		✅ in 7s	⏭️	⏭️

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