Partial fix for nested keypaths through @CompositeOptionalParent properties

[gwynne]

These changes are now available in 1.50.4

As with @CompositeParent, keypaths through @CompositeOptionalParent can refer to properties of the target model's composite IDValue. However, the subscript(dynamicMember:) accessor of @CompositeOptionalParent which enables this functionality returns an optional value, and the result is always nil if the relation has not been loaded.

This was caused by an oversight during the original implementation of the composite relation properties: The accessor should not be returning an optional value, nor should the result ever be nil regardless of whether or not the relation is loaded. We can't fix the fact that it returns an optional without breaking public API, so callers still have to add a ! in the nested keypaths. But we can at least make the accessor useful by fixing it to never return nil, which is what this PR does. A test is also included.

Also updates the CI while we're at it.

Example (omits some boilerplate for brevity):

final class ParentModel: Model, @unchecked Sendable {
    final class IDValue: Fields, @unchecked Sendable {
        @Field(key: "foo")
        var foo: String
    }

    @CompositeID var id: IDValue?
}

final class ChildModel: Model, @unchecked Sendable {
    @ID
    var id: UUID?

    @CompositeOptionalParent(prefix: "parent")
    var parent: ParentModel?
}

// This would previously print `nil`.
print(ChildModel()[keyPath: \ChildModel.$parent.$foo?.queryablePath][0]) // prints Optional("foo")

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 24.26%. Comparing base (4f886eb) to head (dfb9f50).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/FluentBenchmark/Tests/SchemaTests.swift	0.00%	2 Missing ⚠️
...ources/FluentBenchmark/Tests/SoftDeleteTests.swift	0.00%	1 Missing ⚠️
Sources/FluentBenchmark/Tests/UniqueTests.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #632      +/-   ##
==========================================
+ Coverage   24.22%   24.26%   +0.03%     
==========================================
  Files         149      149              
  Lines        9348     9348              
==========================================
+ Hits         2265     2268       +3     
+ Misses       7083     7080       -3     

Files with missing lines	Coverage Δ
...FluentKit/Properties/CompositeOptionalParent.swift	50.60% <100.00%> (+2.40%)	⬆️
...ources/FluentBenchmark/Tests/SoftDeleteTests.swift	0.00% <0.00%> (ø)
Sources/FluentBenchmark/Tests/UniqueTests.swift	0.00% <0.00%> (ø)
Sources/FluentBenchmark/Tests/SchemaTests.swift	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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