Respect disabled Avalonia name generator

[mklts]

What does this PR do?

Fixes AvaloniaNameGeneratorIsEnabled=false for the Avalonia XAML name generator.

The property is still defined in Avalonia.Generators.props and passed to the source generator through CompilerVisibleProperty, and it is still read by GeneratorOptions. However, after the migration to the incremental source generator pipeline, the value was no longer checked, so setting:

<AvaloniaNameGeneratorIsEnabled>false</AvaloniaNameGeneratorIsEnabled>

did not actually disable the name generator.

This PR restores the missing check and filters out XAML inputs before parsing/generation when the generator is disabled.

Regression
This was introduced in:

fca45c1

Migrate AvaloniaNameSourceGenerator to IIncrementalGenerator (#19216)

Author: @maxkatz6
Author date: 2025-08-13T13:22:14-07:00

Before that commit, the old AvaloniaNameSourceGenerator had an explicit gate:

var options = new GeneratorOptions(context);
if (!options.AvaloniaNameGeneratorIsEnabled)
{
    return null;
}

During the migration to AvaloniaNameIncrementalGenerator, the option continued to be read by GeneratorOptions, but the actual enabled check was not ported to the new incremental pipeline.

Fix
The XAML input filter now returns false immediately when AvaloniaNameGeneratorIsEnabled is disabled:

if (!options.AvaloniaNameGeneratorIsEnabled)
{
    return false;
}

This prevents the name generator from parsing XAML files and from producing generated .g.cs files when disabled, matching the previous behavior.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067531-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]