[release/8.0] Fix OPENJSON postprocessing with split query (#32978)

[roji]

Fixes #32976, backports #32978

Description

The SqlServerJsonPostprocessor introduced in EF 8 did not properly visit the shaper expression (in addition to the query expression), missing necessary rewriting of split queries which are stored there.

Customer impact

Split queries are an important, commonly-used EF mechanism to avoid severe performance issues (e.g. the so-called "cartesian explosion") when querying out related collections.

JSON postprocessing is a query processing step which is necessary when user queries include a JSON collection that requires ordering (or represents a collection of the SQL Server hierarchyid type). This includes any query which composes an order-sensitive operator on top of a JSON column or parameter, e.g.:

var blogs = await context.Blogs.Where(b => b.Tags[0] == "foo").ToListAsync();

The intersection of these two things - split queries and JSON queries with order sensitivity - currently produce incorrect SQL and fail, and this is what's fixed in this PR. This intersection is likely to be not uncommon.

How found

Customer reported on 8.

Regression

No, as far as we're aware the affected scenarios weren't possible in 8.0.

Testing

Tests added.

Risk

Very low, safe one-line fix; quirk added.

[roji]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[SamMonoRT]

@roji - is this ready to add the Servicing Consider label for tactics approval?

[roji]

@SamMonoRT yes... My bad, it should have been there from the start.

[AndriySvyryd]

@roji Please add more background info on why we added SqlServerJsonPostprocessor, what is split query and roughly how likely is it for a user to hit this scenario.

[roji]

@AndriySvyryd done, let me know if that seems sufficient.