Metadata parse-failure diagnostic has no location, so malformed JSON can't be clicked through to the bad line #28

What's wrong

MetadataFile.Deserialize<T> (SourceGeneratorToolkit/MetadataFile.cs, the local ParseFailed function) reports the parse failure with context.Report(parseFailed, FileName, ex.Message). DiagnosticCatalog.Report always reports at Location.None (DiagnosticCatalog.cs:103).

For a JsonException, the exception already carries LineNumber and BytePositionInLine. MetadataFile also already has the text and a LocationAt(index, length) helper, which it uses to make its other diagnostics navigable. The toolkit's most common diagnostic is the one that isn't navigable, even though the README advertises navigable diagnostics.

Failure scenario

A things.json containing {"things": [1,,2]} on line 3 produces a MetadataParseFailed diagnostic with no file or line in the IDE error list. The only pointer is the "LineNumber: 2 | BytePositionInLine: …" text buried in the message, and double-clicking goes nowhere.

Suggested fix / acceptance criteria

• In the JsonException branch, when ex.LineNumber has a value and the text is available, compute an offset from ex.LineNumber and ex.BytePositionInLine and report with ReportAt at that location.
  • BytePositionInLine counts UTF-8 bytes, so convert it to a character offset on non-ASCII lines.
  • Clamp the offset to the line length.
• Keep Location.None for the NotSupportedException, InvalidOperationException and null-document cases, and for a JsonException without a line number.
• Test: malformed JSON on line 3 produces a diagnostic whose Location.GetLineSpan() has the metadata file's path and StartLinePosition.Line == 2.