EF-359: Fix Count/LongCount over embedded collections in projections

[ajcvickers]

Supersedes #337

b.Posts.Count(p => p.Rank > 0) and b.Posts.Count() inside a Select projection over an owned (embedded) collection threw InvalidOperationException at translation time in every MongoQueryMode, because MongoProjectionBindingExpressionVisitor had no case for the lowered Queryable.Count(Queryable.AsQueryable(EF.Property(...))) shape and fell into the generic fallback, which tried to visit the predicate lambda's own parameter as an unbound reference.

Bind Count/LongCount (with or without a predicate) to the original method-call subtree instead of the materializing shaper, so it stays push-down-able to the driver's own LINQ v3 translation rather than materializing owned entities (which EF Core's tracking-materializer rejects without an owner in the result).

That push-down path also has a null-safety gap for the bare/unfiltered form: the driver renders it as a server-side $size, which throws on a missing/null stored array (unlike the predicated form, which renders as a null-tolerant $map/$sum). Add a narrowly-scoped rewrite in
MongoEFToLinqTranslatingExpressionVisitor that null-guards genuinely embedded navigations with $cond, without touching other AsQueryable(...) shapes (e.g. IGrouping sources in GroupBy/Union pipelines).

This also fixes and subsumes EF-357 (bare/arithmetic-wrapped Count over an embedded collection), including a tracking-query case EF-357's own fix did not handle (its client-side materialization approach hits the same owned-entity-without-owner rejection under a tracking query).

[Copilot]

Pull request overview

Fixes server-side translation of Count and LongCount projections over embedded collections.

Changes:

• Binds collection counts directly for LINQ v3 translation.
• Adds null guards for unfiltered counts.
• Adds functional coverage for filtered, arithmetic, tracking, and null cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
MongoProjectionBindingExpressionVisitor.cs	Binds embedded collection count projections.
MongoEFToLinqTranslatingExpressionVisitor.cs	Rewrites unfiltered counts with null handling.
OwnedEntityTests.cs	Adds embedded collection count regression tests.

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[damieng]

@copilot resolve the merge conflicts in this pull request

[damieng]

1. Interface-typed owned collections still throw. Activator.CreateInstance in MongoEFToLinqTranslatingExpressionVisitor.cs can't build IList<T>, ICollection<T>, IEnumerable<T> or T[]. A repro threw MissingMethodException on EF10 and EF8. The suggested fix is Array.CreateInstance(elementType, 0) for those types, plus an IList<T> test.
2. Entity plus Count still fails. Select(b => new { b, N = b.Posts.Count }) throws The property 'Blog.N' could not be found, because the mixed-projection visitor doesn't recognise the new Count leaf.
3. Nested owned Count still fails (nit). b.Posts.Select(p => p.Tags.Count()) throws the "owned entity without a corresponding owner" error.

All three threw before the PR too, so none is a regression but we probably want this to be comprehensive.

[ajcvickers]

@damieng Comment 3 is complicated to fix and not a regression. If it's really more than just a nit, then we should file an issue for it.

[ajcvickers]

@damieng Looks like I still need a re-review of this.