better perf by avoiding redundant casts

[SimonCropp]

No description provided.

[Copilot]

Pull request overview

This PR performs small, targeted micro-optimizations across SqlClient hot paths by replacing repeated is checks + explicit casts with C# pattern matching (is T t) to avoid redundant casts/unboxing and simplify code.

Changes:

• Replace if (x is T) { (T)x ... } patterns with if (x is T t) { t ... } in multiple locations.
• Simplify a couple of ConvertTo(...) TypeConverter implementations by combining type checks and casts.
• Minor readability cleanup in exception handling and bulk-copy XML handling.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParser.cs	Uses pattern-matching locals to avoid repeated casts/unboxing while writing/serializing values (RPC params, bulk copy, and serialization paths).
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlUtil.cs	Simplifies SqlException detection/cast when building error arrays.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlParameter.cs	TypeConverter ConvertTo now pattern-matches SqlParameter once before calling instance descriptor conversion.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlConnectionStringBuilder.cs	TypeConverter ConvertTo now pattern-matches SqlConnectionStringBuilder before instance descriptor conversion.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlCommand.cs	Avoids duplicate cast by binding SqlString via pattern matching in ANSI size computation logic.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlBulkCopy.cs	Avoids duplicate cast when converting XmlReader to XmlDataFeed for XML/JSON bulk copy values.