feat: add Repr instance for Vector

[Seasawher]

This PR renders Vector values using #v[...] literal notation instead of exposing their underlying structure representation, making evaluated vectors more concise and readable. It replaces the derived Repr instance.

see Zulip: #lean4 > Make the Repr instance for Vector to the literal notation

[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 a2c74ef85a8212503551663871b90d0d51b7417f --onto 3259610687883ec1ea48c481aba2469f2f83facf. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-25 08:00:56)

[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 a2c74ef85a8212503551663871b90d0d51b7417f --onto 156bdc91d998a1156341d899f43681190e74d2b8. You can force reference manual CI using the force-manual-ci label. (2026-07-25 08:00:57)