Avoid using SqlXml for serialization

[AndriySvyryd]

Follow-up to #38462 improving performance

[Copilot]

Pull request overview

This PR is a follow-up to #38462, changing SQL Server xml parameter handling to avoid wrapping XML in SqlXml (and its extra buffering) by sending an XmlReader directly to SqlClient, improving performance while keeping the SqlDbType.Xml path.

Changes:

• Switch SqlServerStringTypeMapping to set SqlParameter.Value to an XmlReader for xml store type instead of SqlXml.
• Update SqlServerQueryStringFactory so ToQueryString() can render xml parameters whose value is an XmlReader.
• Adjust SQL Server tests to validate the new XmlReader parameter behavior and update related comments.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
test/EFCore.SqlServer.Tests/Storage/SqlServerTypeMappingTest.cs	Updates unit test expectation from SqlXml to XmlReader for xml parameters.
test/EFCore.SqlServer.FunctionalTests/BuiltInDataTypesSqlServerTest.cs	Updates functional test comments around the xml parameter path exercised by the test.
src/EFCore.SqlServer/Storage/Internal/SqlServerStringTypeMapping.cs	Sends xml string values as a streaming XmlReader rather than buffering via SqlXml.
src/EFCore.SqlServer/Query/Internal/SqlServerQueryStringFactory.cs	Adds XmlReader handling to reconstruct a printable canonical xml literal for ToQueryString().

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.