Add repro test for duplicate split migration names

[jechol]

Adds a failing regression test for duplicate migration names/modules when a single generation run is split into multiple migration files.

This is a minimal reproduction of #723.

This repro uses a concurrent custom index together with an explicit name: "repro_case".

The generated files end up with the same explicit suffix:

• 20260320063017_repro_case.exs
• 20260320063018_repro_case.exs

and both define the same migration module:

• AshPostgres.TestRepo.Migrations.ReproCase

That means split migrations are not deriving unique names/modules from the provided base name.

This PR is intended as a minimal repro only.

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