Support C# required keyword in source generation and reflection

[fdcastel]

Summary

Types using the C# required keyword (C# 11+) fail to compile when registered with the source generator because the generated deserialization code emits new T() without setting required members, producing CS9035 errors. The reflection path also doesn't detect required members for validation.

This PR fixes both the source generator and the reflection converter to fully support C# required members.

Problem

public class DnsMonitorConfiguration
{
    public required string Host { get; set; }
    public TimeSpan Timeout { get; set; }
}

// Source generator emits:
var instance = new DnsMonitorConfiguration();  // CS9035: required member 'Host' must be set
instance.Host = __host_value;

Solution

Source Generator (YamlSerializerContextGenerator.cs)

1. Detect required modifier — CreateMemberModel() now checks IPropertySymbol.IsRequired and IFieldSymbol.IsRequired in addition to [YamlRequired] and [JsonRequired] attributes.

2. Route through initializer path — Types with required keyword members are routed through the constructor/object-initializer code path (same as init-only properties), avoiding the bare new T() that causes CS9035.

3. Use default! as dead-code fallback — Required-keyword members in the object initializer use default! instead of __defaults.Member, since the required-member validation will throw before that fallback is ever observed.

4. __defaults construction — When pure init-only members coexist with required keyword members, the defaults instance includes required members in its own initializer with default!.

Generated code (after fix):

var instance = new DnsMonitorConfiguration()
{
    Host = __member0_Host_seen ? __member0_Host : default!,
};
instance.Timeout = __member0_Timeout;

if (!__required0_Host)
{
    var missing = new List<string>();
    if (!__required0_Host) missing.Add("Host");
    throw YamlThrowHelper.ThrowMissingRequiredMembers(...);
}

Reflection (YamlObjectConverter.cs)

• IsRequired() now checks for System.Runtime.CompilerServices.RequiredMemberAttribute on .NET 8+ targets, detecting the C# required keyword at runtime.

Test Coverage

35 new tests covering:

• required properties with set and init accessors
• Mixed modes: required keyword + [YamlRequired] + [JsonRequired]
• Records with required members
• Value type required properties
• Inheritance (required in base + derived)
• Naming policies (camelCase)
• Custom options via CreateOptions()
• Missing required member validation (single and multiple)
• Both reflection and source-generated modes

All 626 tests pass (591 existing + 35 new).

[fdcastel]

Sorry for the messy commit history -- I’ll clean it up soon.

[fdcastel]

Commit history rewritten.

Just running the final tests before taking it out of draft.

[fdcastel]

Just for the record: I’m hosting some temporary, unofficial builds here. Source: here.

These builds include all currently submitted PRs (except the ones that were rejected, like #136).