Checker: do not copy inferred file signature

[auduchinok]

Continues #20478.

Removes extra copying of inferred signature which prevents extra work and reuses the symbols from the analysis in the signature.

[github-actions]

❗ Release notes required

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---

✅ Found changes and release notes in following paths:

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Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md	No current pull request URL (#20524) found, please consider adding it

[github-actions]

🔍 Tooling Safety Check — Affects-Design-Time
Affects-Design-Time: Changes background and transparent compiler analysis paths.

Generated by PR Tooling Safety Check · gpt56 835.2K · ◷

[T-Gro]

🤖 🕵️ AI review — verify independently.

[T-Gro]

🤖 🕵️ FileSignature loses the declaring entity of nested types from earlier files; PartialAssemblySignature returns the correct parent.

// First.fs
module First
module Nested =
    type R = { Value: int }

// Second.fs
module Second
let r: First.Nested.R = { Value = 1 }

// After checking Second.fs:
let owner =
    results.FileSignature.FindEntityByPath ["Second"]
    |> Option.get
    |> fun e -> e.MembersFunctionsAndValues
    |> Seq.find (fun v -> v.DisplayName = "r")
    |> fun v -> v.FullType.TypeDefinition.DeclaringEntity
    |> Option.map (fun e -> e.FullName)
// None; expected Some "First.Nested"