[g31pranjal]

Implements SyntheticRecordType.expand for JoinedRecordType, which until now threw. #4626 added expand and implemented it for UnnestedRecordType; this does the same for joins.

A joined record type expands to one scan per constituent and one equality predicate per join:

• Each constituent is the expansion of its stored record type, a type-filtered scan, flowed whole as a result column named after the constituent. A type joined to itself gets one scan per copy, each with its own quantifier, so the join can relate the copies.
• Each join becomes left = right, with each side's key expression translated into a value over its own constituent's quantifier. Nested join fields become nested field values.
• A constituent with no join leaves the pair unconstrained, which is the same cross product the joined record type itself produces.

Outer-joined constituents are rejected with UnsupportedOperationException. An outer join keeps a record that has no partner, and an inner equality between two scans would drop it.

This is groundwork: nothing calls expand yet. It is what lets an index on a joined record type become a match candidate, so that a query performing the same join by hand can be answered from the index.

UnnestedRecordTypeExpansionTest is renamed to SyntheticRecordTypeExpansionTest, which now tests the expansion of both kinds of synthetic type. Its test expecting a joined type to throw is replaced by tests of a two-way join, a self-join, a three-way cyclic join (one equality per join), a join whose constituents are named in the opposite order to their declaration, a nested join field, a constituent with no join, and outer-join rejection.