refactor(psql/mm): replace WHEN clause mods with fluent chain API

[atzedus]

Sorry for the back-and-forth on this - after the initial merge implementation I went back and re-read the MERGE statement syntax more carefully:

WHEN MATCHED [ AND condition ] THEN { merge_update | merge_delete | DO NOTHING }
WHEN NOT MATCHED BY SOURCE [ AND condition ] THEN { merge_update | merge_delete | DO NOTHING }
WHEN NOT MATCHED [ BY TARGET ] [ AND condition ] THEN { merge_insert | DO NOTHING }

It became clear that each WHEN MATCHED / WHEN NOT MATCHED clause is a self-contained, standalone expression — the condition (AND ...) and the action (THEN ...) are integral parts of that single clause, not independent modifiers applied to some external builder. The previous functional-options approach (WhenMatched(And(...), ThenUpdate(...))) obscured this structure by treating the condition and action as separate, unrelated mods passed into a bag.

Switching to a fluent chain makes the code mirror the SQL grammar directly:

// Before
mm.WhenMatched(
    mm.And(psql.Quote("s", "quantity").GT(psql.Arg(0))),
    mm.ThenUpdate(
        mm.SetCol("quantity").ToExpr(psql.Quote("s", "quantity")),
    ),
)

// After
mm.WhenMatched().
    And(psql.Quote("s", "quantity").GT(psql.Arg(0))).
    ThenUpdate(
        mm.SetCol("quantity").ToExpr(psql.Quote("s", "quantity")),
    )

Changes

• WhenMatched(), WhenNotMatched(), WhenNotMatchedByTarget(), WhenNotMatchedBySource() now take no arguments and return a chain type
• WhenMatchedChain (for MATCHED / NOT MATCHED BY SOURCE) exposes .And(), .ThenUpdate(), .ThenDelete(), .ThenDoNothing()
• WhenNotMatchedChain (for NOT MATCHED [BY TARGET]) exposes .And(), .ThenInsert(), .ThenInsertDefaultValues(), .ThenDoNothing()
• Removed standalone And, ThenDoNothing, ThenDelete, ThenUpdate, ThenInsert, ThenInsertDefaultValues functions and the WhenClause type
• All existing tests updated and passing