Cabal multi-repl fails with Custom build type and the error message is not helpful

**Describe the bug**

If I understand [this blog post](https://well-typed.com/blog/2024/07/hls-multi/) correctly, cabal 3.12.1.0 is supposed to have multi-component support. Unfortunately, trying to load multiple components with HLS still produces an error which contains this line:

```
The 'repl' command does not support multiple targets at once.
```

**To Reproduce**
Steps to reproduce the behavior:

* clone https://github.com/agda/agda.git
* open `src/full/Agda/Benchmarking.hs` (in VSCode)
* wait for it to load
* open `test/Bugs/Tests.hs`
* see error

**Expected behavior**

No error.

**System information**
- Operating system: NixOS
- `cabal`, `ghc` versions:

```
$ cabal --version
cabal-install version 3.12.1.0
compiled using version 3.12.1.0 of the Cabal library

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 9.6.6

$ haskell-language-server-wrapper --version
haskell-language-server version: 2.9.0.0 (GHC: 9.6.6) (PATH: /nix/store/w7h9j7ar2q1zk3dldh5adhbs85vm9kir-haskell-language-server-2.9.0.0/bin/haskell-language-server-wrapper)
```

Reproduce the failure with the Agda files and the listed Cabal, GHC, and HLS versions. Trace how Cabal handles multiple REPL targets for a Custom build type; done means loading both components no longer produces the multiple-targets error.

Written by the indexing model from the issue text.