Implement UIA TextPattern for TextBox on Windows

[aryanchoudharypro]

What does the pull request do?

TextBox on Windows previously only exposed ValuePattern, so UI Automation clients (screen readers such as NVDA) had no way to track the caret or selection while editing — focusing the control announced the whole text once, but arrowing through it, selecting, or editing gave no further feedback.

This adds a full TextPattern/TextRangeProvider implementation for TextBox on Windows:

• New platform-agnostic Avalonia.Automation.Provider.ITextProvider/ITextRangeProvider contracts, implemented by TextBoxAutomationPeer and a nested TextRange type against TextBox/TextPresenter/TextLayout (character/word/line navigation, selection, bounding rectangles).
• AutomationPeer gains TextSelectionChanged/TextChanged events, raised by TextBoxAutomationPeer and coalesced so a single edit produces one UIA event rather than one per touched property.
• Win32 glue (AutomationNode.Text.cs, Win32TextRangeProvider) wires the above into the existing GeneratedComInterface-based UIA provider, wrapping fresh ITextRangeProvider instances since, unlike AutomationNode, they aren't cached per-peer.

Along the way, found and fixed several real bugs in the existing (previously unused/untested) Win32.Automation interop and marshalling code, surfaced only once an out-of-process client (NVDA) actually exercised it:

• ITextProvider.RangeFromPoint took two scalar doubles instead of the UiaPoint struct the real UIA ABI expects.
• SafeArrayRef.CreateFromObjects used ComWrappers.TryGetComInstance (only returns a wrapper that already exists, doesn't create one) instead of GetOrCreateComInterfaceForObject, silently leaving uninitialized ArrayPool-rented pointer data in the SAFEARRAY for objects with no prior COM exposure — this reliably crashed the process (UIAutomationCore!ProtobufHelper::WriteTextRangeArray, a null-pointer dereference) as soon as NVDA queried GetSelection().
• Interface-typed SAFEARRAYs were built with SafeArrayCreate instead of SafeArrayCreateEx, so they carried no IID (FADF_HAVEIID) — native code marshaling such an array to an out-of-process client has no way to know which interface each element pointer is.
• ITextRangeProvider.GetAttributeValue returned a plain -1 for unsupported attributes instead of UIA's reserved "not supported" sentinel (UiaGetReservedNotSupportedValue). NVDA reads a plain -1 as a real (truthy) attribute value; for the hyperlink attribute this made a plain TextBox get announced as a link.

What is the current behavior?

TextBox on Windows only implements ValuePattern. Screen readers get a single whole-text announcement on focus and no further caret/selection/edit feedback.

What is the updated/expected behavior with this PR?

TextBox on Windows now implements TextPattern as well. To test: run a Windows build with NVDA (or another UIA-based screen reader) active, focus a TextBox, and confirm caret position, character/word navigation, and selection are announced correctly while typing and arrowing through text.

How was the solution implemented (if it's not obvious)?

Three layers, built and verified in this order:

1. Platform-agnostic layer (Avalonia.Controls) — ITextProvider/ITextRangeProvider interfaces plus the TextBoxAutomationPeer/TextRange implementation. Covered by unit tests, no COM involved.
2. Win32 COM glue (Avalonia.Win32.Automation) — AutomationNode.Text.cs implements the existing (previously unused) UIA.ITextProvider interop interface by delegating to layer 1 via the existing InvokeSync dispatcher-marshaling helpers (now shared via the extracted Win32DispatcherHelper, since Win32TextRangeProvider isn't an AutomationNode and needs the same marshaling). Win32TextRangeProvider wraps a single ITextRangeProvider instance as its UIA.ITextRangeProvider COM counterpart.
3. Marshalling fixes — the four bugs listed above in SafeArrayRef/ITextProvider's interop declaration, none of which were previously exercised by any existing pattern (ISelectionProvider etc. only ever return already-wrapped, long-lived AutomationNode instances, so the TryGetComInstance/missing-IID gaps never surfaced before).

Root-caused the crash and the "announced as a link" misbehavior using cdb (WinDbg's console debugger) attached to the live process, and verified the fix with NVDA actually running against a real build.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None. TextBoxAutomationPeer gains new interface implementations (ITextProvider) but its existing public surface (IValueProvider) is unchanged.

Obsoletions / Deprecations

None.

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0068366-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[aryanchoudharypro]

@cla-avalonia agree

[Gillibald]

This is already part of #20890

[MrJul]

Thank you for your contribution. I'm closing this PR in favor of #20890.