[Repo Assist] Fix #1453: Register CodePagesEncodingProvider on .NET 6+ for code-page encodings

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Closes #1453

Root Cause

On .NET 6+ (the net8.0 design-time target), code-page encodings such as windows-1250 (cp1250) are not registered by default. Encoding.GetEncoding(1250) throws:

No data is available for encoding 1250. For information on defining a custom encoding, see the documentation for the Encoding.RegisterProvider method.

The fix in PR #1503 (closed) correctly identified the problem but added the registration in the wrong location (CsvProvider.fs); encoding resolution actually happens in IO.fs's asyncRead when opening a local file with a non-default encoding.

Fix

In IO.fs, inside asyncRead, wrap the existing encodingStr resolution with a one-time CodePagesEncodingProvider registration, guarded by #if NET6_0_OR_GREATER:

+#if NET6_0_OR_GREATER
+                    Encoding.RegisterProvider(CodePagesEncodingProvider.Instance)
+#endif
                     HttpEncodings.getEncoding encodingStr

CodePagesEncodingProvider.Instance is available in-box on .NET 6+; the netstandard2.0 target is left unchanged (registering there would require the System.Text.Encoding.CodePages package which is not currently a dependency). RegisterProvider is idempotent so calling it per-read is harmless.

Trade-offs

• Only fixes net8.0 target; netstandard2.0 remains limited to standard encodings without the extra package.
• No new dependencies introduced.

Test Status

Build: ✅ dotnet build FSharp.Data.sln -c Release — 0 errors, 23 pre-existing warnings
Tests: ✅ 3,310 passed; 19 failures all pre-existing network failures (HTTP 403 in sandboxed CI) unrelated to this change

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[github-actions]

✅ Pull request created: #1612