Fold single-table filter subqueries into joins on SQLite

[Aykuttonpc]

When a query filters a joined table (an optional navigation with a Where, a SelectMany with a predicate, etc.), EF currently emits a join over a subquery even when the subquery does nothing but filter a single table:

LEFT JOIN (
    SELECT "l0"."Id", "l0"."Name"
    FROM "LevelThree" AS "l0"
    WHERE "l0"."Id" > 5
) AS "l1" ON "l"."Id" = "l1"."OwnerId"

Since the filter references only the inner table, it can move onto the join condition and the subquery can go away:

LEFT JOIN "LevelThree" AS "l1" ON "l"."Id" = "l1"."OwnerId" AND "l1"."Id" > 5

This adds SqliteSubqueryToJoinRewriter, run from SqliteQueryTranslationPostprocessor, which does exactly that: rewrites a LEFT/INNER JOIN over a single-table filter subquery into a join over the table, folding the filter into the ON condition and inlining the subquery's projection.

Where it deliberately does nothing

Working through the test suite surfaced several cases where folding the filter out would change results, so the rewrite skips them:

• Another join references the subquery's alias — a later join keying off this one (nested collection includes) relies on it having filtered its rows first.
• The join key depends on a sibling join — a correlated join chain (e.g. keying off a preceding optional/LEFT JOIN) relies on the current join order.
• A LEFT JOIN projection isn't purely columns — the projected columns must be made nullable under a LEFT JOIN, which only works for bare column references.
• The predicate is only IS NOT NULL checks — that's an optional-entity existence test (table splitting), not a value filter; the containing query uses it to decide whether the entity is present.

Scope

This is implemented in the SQLite provider only. The underlying reasoning is provider-agnostic (it's about relational join semantics), so it could move to the relational layer to benefit all providers, but I kept it local to SQLite so it could be validated against the full SQLite functional suite. Happy to move it up if that's the preferred direction.

Tests

Full SQLite functional suite passes: 38367 tests, 0 failures. 97 query baselines lost their subqueries (net −334 lines in the affected test files, e.g. GearsOfWarQuerySqliteTest shrank noticeably). No data or nullability regressions — each skip condition above was added specifically because a test showed the rewrite was otherwise changing results.

Part of #33745