…te-type Try-methods

PR #642 annotated all Try* out-parameters with [MaybeNullWhen(false)] on a
non-nullable T, which is the correct pattern for unconstrained generic type
parameters (as in Dictionary<TKey,TValue>.TryGetValue) but not for concrete
reference types. For concrete types, the BCL convention (e.g. Uri.TryCreate)
is [NotNullWhen(true)] out T? - this also gives callers a compiler warning
if they dereference the out value without checking the return value first,
which MaybeNullWhen(false) on non-nullable T silently allows.

Updates IHelperResolver, IObjectDescriptorProvider (and implementers),
IFormatterProvider (and implementers), TypeExtensions.IsAssignableToGenericType,
and BlockAccumulatorContext.IsDetachedClosingElement. Generic-TValue Try-methods
(LookupSlim, DictionarySlim, FixedSizeDictionary, CascadeIndex, ObservableIndex)
are left as-is since they correctly mirror the BCL generic pattern.

Fixes #654

…es PR

PR #642 touched two public readonly array field declarations (adding `?`
annotations) without changing their design, which caused SonarCloud to
flag S3887 ("non-private readonly field exposes mutable array") as new
Bug-type issues on both lines, dropping the reliability rating from A to B.

- Closure.A: made internal, since Closure's constructor is already
  internal and the field is unreachable by external consumers. Added
  InternalsVisibleTo("Handlebars.Test") for the one test that reads it
  directly, and pass BindingFlags.NonPublic to the reflection GetField
  call that resolves this field for compiled closure expressions.
- PathInfo.Segments: left public with a NOSONAR suppression instead,
  since PathInfo instances do reach public surfaces (HelperOptions,
  PathExpression, etc.) and the field has been public since 2020 -
  flipping it to internal would be a real breaking change.