feat: add a Decidable instance for bif (cond)

[marcusrossel]

This PR adds a missing Decidable instance for bif (cond) expressions, analogous to the existing instance for if (ite) expressions.

Discussed in https://leanprover.zulipchat.com/#narrow/channel/270676-lean4/topic/Decidable.20bif/near/621670745.

[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 67526ae54d733251644d5cebb9fa5a77318fecc4 --onto c632a0a0e434a951cdcf61bb4da3344abadd5587. You can force Mathlib CI using the force-mathlib-ci label. (2026-09-04 11:46:19)

[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 67526ae54d733251644d5cebb9fa5a77318fecc4 --onto 58774429865502f05c63239266aac30ef1e91ef7. You can force reference manual CI using the force-manual-ci label. (2026-09-04 11:46:21)

[TwoFX]

Thanks!