feat: upstream unusedDecidableInType linter

[wkrozowski]

This PR upstreams unusedDecidableInType linter from mathlib.

Stacked on top of #11313.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 355dca6f57c6fec817d67765c566dc6ba6ac84bd --onto 5d5642107d0433519265f155ddbfbfb98007a80b. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-08 10:18:48)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 48ad8401cd5624c944890893a38057ae4920e8ef --onto a71f158f7bd96ff9ea846f7ff4cc658de3c8b0f9. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-11 14:26:13)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 355dca6f57c6fec817d67765c566dc6ba6ac84bd --onto 5d5642107d0433519265f155ddbfbfb98007a80b. You can force reference manual CI using the force-manual-ci label. (2026-05-08 10:18:50)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 48ad8401cd5624c944890893a38057ae4920e8ef --onto a71f158f7bd96ff9ea846f7ff4cc658de3c8b0f9. You can force reference manual CI using the force-manual-ci label. (2026-05-11 14:26:15)

[wkrozowski]

!bench

[leanprover-radar]

Benchmark results for 18ac8ff against 48ad840 are in. No significant results found. @wkrozowski

• 🟥 build//instructions: +4.4G (+0.04%)

Small changes (1🟥)

• 🟥 build/module/Lean.Expr//instructions: +151.4M (+1.19%) (reduced significance based on *//lines)