Revert #138 CanConvert widening; replace with workaround example

[Copilot]

#138 widened UnionConverter<'T>.CanConvert to accept nested case types (t.DeclaringType = typeof<'T>) to support serializing boxed DUs. This is not recommended practice from an STJ perspective and shouldn't live in the library.

• src/FsCodec.SystemTextJson/UnionConverter.fs: Revert CanConvert to pre-Fix UnionConverter.CanConvert for boxed multi-case DU #138 form: t = typeof<'T> && FsCodec.Union.isUnion t
• tests/.../UnionConverterTests.fs: Replace direct UnionConverter usage with a local BoxedUnionConverterFactory<'T> that carries the widened CanConvert logic, serving as a copyable workaround example

Users needing boxed DU serialization can derive a custom factory:

type BoxedUnionConverterFactory<'T>() =
    inherit JsonConverterFactory()
    override _.CanConvert t = (t = typeof<'T> || t.DeclaringType = typeof<'T>) && FsCodec.Union.isUnion t
    override _.CreateConverter(_t, _options) = UnionConverter<'T>()

let opts = JsonSerializerOptions()
opts.Converters.Add(BoxedUnionConverterFactory<MyUnion>())
JsonSerializer.Serialize(Case1 42 :> obj, opts)

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[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[bartelink]

@copilot please add a terse summary to the Removed section of the changelog Unreleased area, citing #144

[Copilot]

@copilot please add a terse summary to the Removed section of the changelog Unreleased area, citing #144

Added in aa6f3e4 under ### Removed in the Unreleased section.