Add async methods to OpenXmlReader

[Copilot]

OpenXmlReader had no async API despite OpenXmlWriter having full async support. This adds parity by introducing ReadAsync, ReadFirstChildAsync, ReadNextSiblingAsync, and SkipAsync.

Changes

• OpenXmlReader (base class) — virtual async methods that fall back to their sync counterparts (same pattern as OpenXmlWriter); guarded by #if TASKS_SUPPORTED so net35/net40 are unaffected
• OpenXmlPartReaderOptions — new Async property (mirrors OpenXmlPartWriterSettings.Async); propagated through UpdateForPart and forwarded to XmlReaderSettings.Async
• OpenXmlPartReader — truly async overrides backed by XmlReader.ReadAsync/SkipAsync/MoveToContentAsync; private async helpers mirror the existing sync MoveToNextElement/MoveToFirstChild/InnerSkip/ReadRoot logic
• XmlConvertingReader — overrides ReadAsync, SkipAsync, MoveToContentAsync to delegate to BaseReader (the wrapper would otherwise fall through to the throwing base XmlReader default)
• Public API — unshipped entries added for netstandard2.0, net6.0, net8.0, net10.0

Usage

// Enable async mode when constructing the reader
using var reader = new OpenXmlPartReader(part, new OpenXmlPartReaderOptions { Async = true });

while (await reader.ReadAsync())
{
    if (reader.IsStartElement)
    {
        // navigate
        await reader.ReadFirstChildAsync();
        await reader.ReadNextSiblingAsync();
        await reader.SkipAsync();
    }
}

OpenXmlDomReader works with the same async API via the base-class fallbacks (no Async flag required).

[github-actions]

Test Results

    51 files   -     29      51 suites   - 29   52m 4s ⏱️ - 42m 23s
 2 091 tests +    13   2 088 ✅ +    13   3 💤 ± 0  0 ❌ ±0 
24 404 runs   - 13 769  24 377 ✅  - 13 754  27 💤  - 15  0 ❌ ±0 

Results for commit 1a8cc72. ± Comparison against base commit 7d1bd90.

[mikeebowen]

@copilot fix the build errors from the previous build

[Copilot]

@copilot fix the build errors from the previous build

Fixed in 94cd2ca. The build failures were SA1515 StyleCop errors in OpenXmlReaderAsyncTest.cs — single-line comments at lines 105 and 107 were not preceded by blank lines. Added the required blank lines before each comment.

[mikeebowen]

@copilot refactor the tests to use using statements

[Copilot]

@copilot refactor the tests to use using statements

Done in 59c3b0a. All tests now use traditional using (...) { } blocks with braces, matching the pattern in the existing OpenXmlReaderTest.cs and OpenXmlWriterTest.cs files.

[mikeebowen]

@copilot refactor FEATURE_ASYNC_SAX_XML to be TASKS_SUPPORTED

[Copilot]

@copilot refactor FEATURE_ASYNC_SAX_XML to be TASKS_SUPPORTED

Done in 7aa9cc0. Renamed FEATURE_ASYNC_SAX_XML to TASKS_SUPPORTED in all 10 files: Directory.Build.props, OpenXmlReader.cs, OpenXmlPartReader.cs, OpenXmlPartReaderOptions.cs, OpenXmlWriter.cs, OpenXmlPartWriter.cs, OpenXmlPartWriterSettings.cs, XmlConvertingReader.cs, and both test files.