fix: correct EnvChange string-type detection in TdsDecoder

Types 5,6 are DWORD pairs (binary), not Unicode strings.
Type 9 (commit tx), 12 (defect tx) are binary transaction descriptors.
Types 19,20 are routing/reserved binary data.

Only types 1-4 (database/language/charset/packetsize) and
13 (log shipping), 18 (user instance) use B_VARCHAR format.

This caused EndOfStreamException during BeginTransaction_CommitAsync
because COMMIT (type 9) has an 8-byte binary old-value but the code
tried to read 16 bytes (8 × 2) as UTF-16.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vkuttyp

src/SqlDotnetty.MsSql/Tds/TdsDecoder.cs

@@ -237,7 +237,7 @@ private static TdsEnvChange ReadEnvChange(BinaryReader r)
         // String-value ENVCHANGE types: 1-6, 9, 12, 13, 19, 20.
         // Each has: newLen(1 byte = #Unicode chars) + newValue + oldLen(1) + oldValue.
         // Binary types (7=collation, 8=begin tx, 10-11=commit/rollback, etc.) use byte arrays.
-        bool isStringType = changeType is >= 1 and <= 6 or 9 or 12 or 13 or 19 or 20;
+        bool isStringType = changeType is >= 1 and <= 4 or 13 or 18;
         if (!isStringType)
         {
             // Binary ENVCHANGE: newLen(1 byte) + newValue(newLen bytes) + ...