[Repo Assist] eng: fix stale FCS VersionOverride; replace Enumerable.Cast with Seq.cast

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🤖 This is an automated pull request from Repo Assist.

Two small engineering quality improvements — one build-system fix (Task 4) and one code idiom improvement (Task 5).

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1. Fix stale FSharp.Compiler.Service VersionOverride (Task 4 — Engineering Investment)

Root cause

A dependabot PR (commit 0c31fe2) bumped FSharp.Compiler.Service to 43.12.201 and added VersionOverride="43.12.201" to FSharp.Formatting.Markdown.fsproj. A subsequent dependabot PR then bumped the central pin in Directory.Packages.props to [43.12.202], but the VersionOverride was not updated. This created a version conflict: the FSharp.Formatting.Common project (a ProjectReference from Markdown) resolved 43.12.202, while the direct PackageReference with the stale VersionOverride tried to pull 43.12.201. The result was a NU1605 ("Detected package downgrade") build warning on every restore.

Fix

Remove the VersionOverride attribute, letting the package use the central pin from Directory.Packages.props (43.12.202) consistently across the solution.

-    <PackageReference Include="FSharp.Compiler.Service" VersionOverride="43.12.201" />
+    <PackageReference Include="FSharp.Compiler.Service" />

Effect: NU1605 warning eliminated; FCS version is now consistent across all projects in the solution.

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2. Replace Enumerable.Cast<XAttribute> with Seq.cast<XAttribute> (Task 5 — Coding Improvement)

In MarkdownUtils.fs, the single use of System.Linq.Enumerable.Cast<XAttribute> can be replaced with the idiomatic F# equivalent Seq.cast<XAttribute>. This allows open System.Linq to be removed from the file — a minor clarity improvement that reduces the surface of imported namespaces.

-open System.Linq
 ...
-                enumerable |> Enumerable.Cast<XAttribute> |> Seq.toArray
+                enumerable |> Seq.cast<XAttribute> |> Seq.toArray

Semantics are identical: both cast the untyped IEnumerable to seq<XAttribute>.

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Changes

File	Change
src/FSharp.Formatting.Markdown/FSharp.Formatting.Markdown.fsproj	Remove stale VersionOverride
src/FSharp.Formatting.Markdown/MarkdownUtils.fs	Remove open System.Linq; use Seq.cast
RELEASE_NOTES.md	Changelog entries

Test Status

⚠️ Infrastructure: dotnet build fails in the sandbox environment with "Creating directory obj/..." errors — this is a pre-existing sandbox limitation, not caused by my changes.

• The VersionOverride removal was verified: dotnet restore on the Markdown project now completes cleanly with zero NU1605 warnings.
• Seq.cast<'T> is the documented F# equivalent of Enumerable.Cast<'T> — the semantics are identical.

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