[Repo Assist] Regression test for idempotency bug with NUnit Assert.That lambda (#1740)

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🤖 This is an automated PR from Repo Assist.

Closes #1740

Summary

Adds a regression test for the idempotency bug reported in #1740, where formatting code like:

Assert.That(
    Assert.Throws(fun () -> FooFooFooFooFooFoo.BarBar.dodododo filesfiles [] outoutout |> ignore)
        .Message,
    Is.EqualTo(message)
)

...would produce a different result on second formatting (non-idempotent).

Root Cause

The bug involved incorrect indentation of a chained .Message property access after a multi-line lambda argument to Assert.Throws. The formatter was moving .Message to an inconsistent position on the second pass.

Fix

The bug is already fixed in the current codebase. The formatter now correctly collapses the expression to:

Assert.That(
    Assert.Throws(fun () -> FooFooFooFooFooFoo.BarBar.dodododo filesfiles [] outoutout |> ignore).Message,
    Is.EqualTo(message)
)

...and this output is stable across multiple formatting passes.

Test Status

✅ New test idempotency problem when formatting NUnit Assert.That with lambda, 1740 passes.
✅ No existing tests broken.

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Commit pushed: f499bd8

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