[#14805] feat: special case single-child nodes in DiscrTree

[downstream-lean4]

This is the adaptation PR for leanprover/lean4#14805.

[downstream-lean4]

Build report for change node to chain in DiscrTree test output

Turned green:

Repo	Critical	Build	Test	Lint
mathlib4	✅	✅ in 220s	✅ in 45s	✅ in 90s
reference-manual	✅	✅ in 16s	⏭️	⏭️
cslib		✅ in 5s	✅ in 8s	✅ in 3s
repl		✅ in 1s	✅ in 56s	⏭️

Stayed green

Repo	Critical	Build	Test	Lint
aesop	✅	✅ in 7s	✅ in 5s	⏭️
batteries	✅	✅ in 5s	✅ in 4s	✅ in 2s
import-graph	✅	✅ in 2s	✅ in 4s	⏭️
lean4-cli	✅	✅ in 1s	✅ in 0s	⏭️
plausible	✅	✅ in 1s	✅ in 2s	⏭️
ProofWidgets4	✅	✅ in 3s	✅ in 1s	⏭️
quote4	✅	✅ in 2s	✅ in 1s	⏭️
BibtexQuery		✅ in 1s	⏭️	⏭️
comparator		✅ in 2s	⏭️	⏭️
doc-gen4		✅ in 3s	⏭️	⏭️
illuminate		✅ in 4s	✅ in 11s	⏭️
lean4-unicode-basic		✅ in 2s	⏭️	⏭️
lean4export		✅ in 0s	✅ in 7s	⏭️
LeanSearchClient		✅ in 1s	✅ in 0s	⏭️
leansqlite		✅ in 4s	✅ in 17s	⏭️
verso		✅ in 25s	✅ in 93s	⏭️
verso-slides		✅ in 30s	✅ in 6s	⏭️
verso-web-components		✅ in 10s	⏭️	⏭️

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