[Repo Assist] Add regression test for index-without-dot with variable key and unit arguments, 2519

[github-actions]

🤖 This is an automated PR from Repo Assist.

Closes #2519

Summary

Adds a regression test for the specific scenario reported in issue #2519: an index-without-dot expression using a variable key, followed by two unit arguments, was being formatted with spurious spaces:

// Before fix (wrong):
dict [ key ] () ()

// After fix (correct):
dict[key] () ()

This was fixed by PR #3259 ("Fix index-without-dot followed by application arguments"). The test ensures this specific pattern from the issue report stays correctly formatted.

Test Added

In IndexSyntaxTests.fs:

[(Test)]
let ``index without dot with variable key followed by two unit arguments, 2519`` () =
    formatSourceString
        """
let test (dict: System.Collections.Generic.IDictionary(string, unit -) unit -> unit>) =
    let key = "foo"
    dict[key] () ()
"""
        config
    |> prepend newline
    |> should
        equal
        """
let test (dict: System.Collections.Generic.IDictionary(string, unit -) unit -> unit>) =
    let key = "foo"
    dict[key] () ()
"""

A CHANGELOG entry has also been added to the [Unreleased] section as requested.

Test Status

• ✅ Build succeeded (dotnet build fantomas.sln)
• ✅ All 2740 tests pass (dotnet test src/Fantomas.Core.Tests/ --no-build)
• ✅ New test passes as expected

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