test: add regression test for #13512

[TWal]

This PR adds a regression test for #13512.

[TWal]

Discussed on Zulip: #lean4 > `simp` / `dsimp` unfold implicit reducible in a match? @ 💬

This regression test is an MWE from a real-world project where I hit this issue with typeclass + grind (as in test_grind_g)

Ping @nomeata

[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 4792cd22887c8b529a351f6563b693426ff2a8f8 --onto 24c48fe0fdbf4bca5b6f907e638732c871dcd2db. You can force Mathlib CI using the force-mathlib-ci label. (2026-06-16 10:03:00)

[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 4792cd22887c8b529a351f6563b693426ff2a8f8 --onto 803553a556fd82fa1060efb0c43eda542130cb16. You can force reference manual CI using the force-manual-ci label. (2026-06-16 10:03:01)