Companion to elixir-ecto/ecto#4765 (fix for elixir-ecto/ecto#4763).

That PR makes interpolated join queries (join: x in ^query) carry their on expression as a BooleanExpr instead of a QueryExpr, since wheres folded into the on may hold subqueries and BooleanExpr already owns them. Per the review discussion, the BooleanExpr is kept in the normalized query rather than converted back, so adapters must accept it.

This relaxes the %JoinExpr{on: %QueryExpr{expr: expr}} pattern matches in the postgres, myxql, and tds connections to the map shape %JoinExpr{on: %{expr: expr}}, accepting both structs. The relaxed patterns are compatible in both directions: this branch passes against ecto 3.14 from hex as well as against the ecto PR branch (ECTO_PATH), so it can ship independently of the ecto release.

Two failure modes without this change (verified by running this suite against the ecto PR branch):

• join/2 raises FunctionClauseError for any interpolated join query — loud.
• the using_join comprehensions (for %JoinExpr{on: %QueryExpr{expr: value}} <- joins, ...) silently drop the join conditions from the WHERE clause of update_all/delete_all — wrong SQL, no error. There was no test coverage for interpolated join queries in update_all/delete_all, so this PR adds it for all three adapters (the new tests pass on current ecto releases too, since the generated SQL is unchanged).

Tests asserting the new subquery-in-join-on SQL itself (e.g. ON p1."id" IN (SELECT ...)) are intentionally left out for now, as they require the unreleased ecto — they can be added once the ecto side ships.

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