Fix XDocument.LoadAsync to not use sync reads

[lilinus]

Fixes #100239

[Copilot]

Pull request overview

This pull request fixes an issue where XDocument.LoadAsync was incorrectly using synchronous reads when processing XML declarations, violating the async contract. The fix ensures that the async code path uses await r.ReadAsync() instead of the synchronous r.Read().

Changes:

• Moved XML declaration handling from shared initialization code into separate sync/async code paths
• Added new XDeclaration.CreateAsync method to handle XML declaration creation asynchronously
• Added comprehensive tests to verify that async methods only call async I/O operations and sync methods only call sync I/O operations

Reviewed changes

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

File	Description
src/libraries/System.Private.Xml.Linq/src/System/Xml/Linq/XDocument.cs	Moved XML declaration handling from InitLoad into Load and LoadAsyncInternal to support separate sync/async code paths
src/libraries/System.Private.Xml.Linq/src/System/Xml/Linq/XDeclaration.cs	Added CreateAsync method to support asynchronous XML declaration creation
src/libraries/System.Private.Xml.Linq/tests/misc/LoadSaveAsyncTests.cs	Added test coverage to verify sync/async I/O patterns are correctly followed

[Copilot]

Extra whitespace before 'standalone' variable name. Should be single space like the lines above.

[stephentoub]

Suggested change

	string? standalone = r.GetAttribute("standalone");
	string? standalone = r.GetAttribute("standalone");

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[stephentoub]

This looks reasonable to me, but @krwq can you take a look, too?

[stephentoub]

Suggested change

	string? standalone = r.GetAttribute("standalone");
	string? standalone = r.GetAttribute("standalone");