Fix ICE in BuildDisposableCleanup when Dispose extension methods are in scope

[Copilot]

BuildDisposableCleanup crashes with an ICE ("Couldn't find Dispose on IDisposable, or it was overloaded") when a C#-style Dispose extension method is in scope alongside the intrinsic IDisposable.Dispose.

open System
open System.Runtime.CompilerServices

type Disposable() =
    interface IDisposable with 
        member _.Dispose() = ()

[<Extension>]
type PublicExtensions =
    [<Extension>]
    static member inline Dispose(this: #IDisposable) = this

let foo() =
    use a = new Disposable()  // ICE here
    ()

Cause

The lookup uses TryFindIntrinsicOrExtensionMethInfo with ResultCollectionSettings.AllResults, which collects both intrinsic and extension methods. The subsequent | [x] -> x match fails when >1 candidate is returned.

Fix

Switch to ResultCollectionSettings.AtMostOneResult. This leverages the existing priority mechanism in AllMethInfosOfTypeInScope — when intrinsic methods are found, extension methods are skipped. Same priority rules as a hand-written (x :> IDisposable).Dispose() call.