test: move DatabaseFixture/EphemeralStationDatabase/KokoroFixture compose plumbing onto ComposeProcess

[genwave-radio]

🧹 Problem

tests/GenWave.Host.Tests/Support/ComposeProcess.cs (added in T591) is meant to be the shared Docker Compose test helper. So far only MediaLibrary.Tests/Support/MigrateShDatabase.cs uses it.

Three older copies of the same plumbing remain:

• tests/GenWave.MediaLibrary.Tests/DatabaseFixture.cs
• tests/GenWave.Host.Tests/Support/EphemeralStationDatabase.cs
• tests/GenWave.Host.Tests/KokoroFixture.cs

✅ Fix

Move all three onto ComposeProcess, with teardown failures surfaced rather than swallowed, and delete the duplicate plumbing.