NoPartialFunctions: fixed bug on object expressions

[webwarrior-ws]

By removing incorrect type cast.

Also added a regression test.

[knocte]

@webwarrior-ws just realised, Seq.cast is a partial function! So NoPartialFunctions should have detected this at SelfCheck, please cook a PR that catches Seq.cast and suggests Seq.choose because Seq.tryCast doesn't exist yet.

E.g.:

// 1. Define the tryCast helper function
let tryCast<'T> (item: obj) =
    match item with
    | :? 'T as casted -> Some casted
    | _ -> None

// 2. Use it with Seq.choose to filter and cast elements
let mixedCollection = [ box "Hello"; box 42; box "World"; box 3.14 ]

let stringsOnly =
    mixedCollection
    | > Seq.choose tryCast<string>

// stringsOnly now contains: seq ["Hello"; "World"]

Then after that cook a PR for https://github.com/fsprojects/FSharpx.Collections to add Seq.tryCast there, and when that gets merged and released, we can change the recommendation in FSharpLint from Seq.choose to FSharpx.Collections.Seq.tryCast