[Repo Assist] fix: re-enable ImplementInterface tests by trimming FCS trailing whitespace

[github-actions]

🤖 This is an automated pull request from Repo Assist.

Partially addresses #1054.

Problem

Since FCS 43.7.200, InterfaceStubGenerator.FormatInterface emits trailing spaces on lines ending with = when followed by a line break (e.g. multiline method stubs). This caused all 40 ImplementInterface tests to fail, so the entire test suite was wrapped in ptestList (skipped) with a comment pointing to the FCS version.

Fix

Post-process the stub generated by FormatInterface to trim trailing whitespace on each line before inserting it into the document:

// FCS may emit trailing spaces on lines ending with `=` followed by a line break.
// Trim each line's trailing whitespace to produce clean output.
let stub =
  stub.Split('\n')
  |> Array.map (fun line -> line.TrimEnd([| ' '; '\t'; '\r' |]))
  |> String.concat "\n"
```

## Test Changes

- Changed `ptestList (nameof ImplementInterface)` → `testList (nameof ImplementInterface)` to re-enable the suite.
- Removed the stale FCS 43.7.200 comment that explained the disabling.
- Applied `dotnet fantomas` formatting to both changed files.

### Still Pending (expected)

Two groups remain as `ptestCaseAsync`:

1. **`cursor position` sub-list** — uses `selectCodeFixWithoutTypeAnnotation`, which requires the "without type annotation" fix path. That path is still disabled pending [dotnet/fsharp#14698](https://github.com/dotnet/fsharp/issues/14698).

2. **`can implement setter when existing getter`** — FCS 43.10.x does not detect an inline getter written as `member _.X with get () = val` as already-implemented via `GetImplementedMemberSignatures`. As a result, both getter and setter are regenerated (which would be a compile error for the user). Marked pending pending investigation of the FCS detection path.

## Test Status

Ran `ImplementInterface` filter against Ionide WorkspaceLoader (BackgroundCompiler + TransparentCompiler):

```
Passed:  40
Ignored: 90   (expected — pending tests for without-type-annotation path and known FCS issue)
Failed:  0
Errored: 0

Build: ✅ dotnet build — 0 warnings, 0 errors
Tests: ✅ 40/40 active tests pass
Format: ✅ dotnet fantomas applied

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