Translate the OptionalTableUpdate built for StatelessSession.upsert and upsertMultiple into a single update statement: filter from the key bindings, $set over value bindings that are both insertable and updatable, $setOnInsert for insertable-only ones (@Column(updatable = false)), upsert: true, multi: false.

Includes groundwork the conflict-clause tickets build on:

• AstUpdateCommand restructured to a list of update statements with upsert and multi flags
• AstDocumentUpdate gains $setOnInsert
• MongoStatement accepts upsert in update statements and extends the JDBC row count to matched plus upserted
• a visitor value descriptor so the dialect requests the upsert translation of visitOptionalTableUpdate, which also serves the forced version-increment update

Rejections are all deferred to performMutation so SessionFactory bootstrap never fails: @Column(insertable = false, updatable = true) attributes (MQL has no matched-path-only write) and entities whose only persistent attribute is the identifier throw bare messages; @Version entities keep throwing, tracked by HIBERNATE-216.