Add processing instruction callbacks

[mamoreau-devolutions]

Summary

TurboXml currently recognizes generic processing instructions (PIs) for well-formedness, but silently discards them. This prevents handler-based consumers from preserving unmodeled XML extension content when they need to capture and later reproduce it.

This PR adds a backward-compatible IXmlReadHandler.OnProcessingInstruction callback so consumers can opt in to handling generic PIs.

API and behavior

void OnProcessingInstruction(
    ReadOnlySpan<char> target,
    ReadOnlySpan<char> data,
    int line,
    int column)

• target is the PI target.
• data is the exact source text after target and before ?>, including any separator whitespace, tabs, and line endings.
• A handler can reconstruct the original PI as <?{target}{data}?>.
• The callback has a default no-op implementation, preserving the existing behavior for handlers that do not need PIs.
• XML declarations remain exclusively routed to OnXmlDeclaration; they are not reported as generic PIs.
• The parser continues to reject malformed PIs and reserved xml-like targets according to its existing validation rules.

The callback follows the parser's existing ReadOnlySpan<char> event model, avoiding string allocation in the parser API.

Why this matters

Consumers that retain unknown or extension XML for later serialization can currently lose a PI during parsing even when they preserve the surrounding elements and attributes. Reporting the PI makes lossless capture and round-tripping possible without changing the default behavior for existing consumers.

Tests

Added regression coverage for:

• exact reconstruction, including whitespace, tabs, and CRLF data;
• PIs with no data;
• PIs inside element content;
• malformed PI rejection; and
• XML declaration routing remaining unchanged.

Validation

cd src
dotnet build -c Release
dotnet test -c Release

[xoofx]

Thanks!