Inconsistent handling of `EnumerableFlag`s & `ExpressibleByArgument `s on the command line

There seems to be inconsistent handling of enum raw values on the command line.

The following code is from `CompletionScriptTests.swift`:

```swift
enum Kind: String, ExpressibleByArgument, EnumerableFlag {
case one, two
case three = "custom-three"
}

@Flag var allowedKinds: [Kind] = []
@Option() var kind: Kind
```

`case three = "custom-three"` from above is considered as `--three` for a flag
from `allowedKinds`, but as `custom-three` as an option value for `--kind`.

This is the case in the 1.7.0 & main completion scripts, in ToolInfoV0 & even in SAP's
command-line argument verification. It seems to me that it should be consistent
throughout; of the 2 options, `custom-three` seems more sensible.

**ArgumentParser version:** `main`
**Swift version:**
swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Target: arm64-apple-macosx15.0

### Checklist
- [x] If possible, I've reproduced the issue using the `main` branch of this package
- [x] I've searched for [existing GitHub issues](https://github.com/apple/swift-argument-parser/issues)

Start with CompletionScriptTests.swift and trace how raw values are rendered for EnumerableFlag cases versus ExpressibleByArgument option values. Compare the corresponding ToolInfoV0 and command-line argument verification paths, then run the relevant tests. Done means the custom-three raw value is handled consistently across completion scripts and argument verification.

Written by the indexing model from the issue text.