feat(analysis): accept targetless, named, and predicated ON CONFLICT

[omer-cengel]

Summary

Upserts can now use the conflict-clause forms that need no new type inference: ON CONFLICT DO NOTHING without a target, ON CONFLICT ON CONSTRAINT name, a predicate on a column-name conflict target, and DO UPDATE ... WHERE. These were previously rejected even though everything they bind can be typed by the existing WHERE rules.

Changes

• Accept ON CONFLICT DO NOTHING without a conflict target on :exec and returning inserts.
• Accept ON CONFLICT ON CONSTRAINT name with DO NOTHING or DO UPDATE; PostgreSQL resolves the constraint name.
• Accept a predicate on a column-name conflict target, typing its placeholders by the WHERE rules against the inserted table under its exposed name (its alias when present).
• Accept DO UPDATE ... WHERE, typing its placeholders against both the inserted table and EXCLUDED; an unqualified column present in both is reported as ambiguous, as PostgreSQL does.
• Bind every conflict-clause placeholder in the order the query writes it.
• Reject DO UPDATE without a conflict target with a message naming ON CONFLICT (id) and ON CONFLICT ON CONSTRAINT name as the fix.
• Add analyzer tests for each form, scope, binding order, and rejection, and a PostgreSQL integration test that runs every form through generated code, including a partial unique index target and a returning DO UPDATE ... WHERE that updates once and then skips.
• Document the accepted targets and actions, the predicate scopes, binding order, returning behavior, and why a conflict-target predicate should be written with literals: PostgreSQL chooses the arbiter index at plan time, so a parameterized predicate stops matching under a generic plan.

Scope and non-goals

• Constraint names are not checked against the schema.
• Conflict-target elements that are expressions or carry a collation or operator class remain rejected with the existing message.
• Predicate columns not compared with a placeholder are left to PostgreSQL, as in WHERE.
• Placeholders the WHERE rules cannot type keep the existing rejection; no new diagnostic is added.
• No SQL parser changes.