fix(postgresql): DROP TABLE ... CASCADE drops dependent views (#4416)

[luongs3]

Fixes #4416.

Problem

When a Postgres schema does DROP TABLE foo CASCADE where a view depends on foo, sqlc removes the table from its in-memory catalog but leaves the dependent view behind. A later CREATE VIEW (or CREATE TABLE) reusing that name then fails with a spurious relation "..." already exists, even though the SQL is valid and runs fine on real Postgres (where CASCADE drops the dependent view).

Reproduction

CREATE TABLE reference_rates (id BIGSERIAL PRIMARY KEY, data_ts TIMESTAMPTZ NOT NULL);
CREATE VIEW vw_reference_rates AS SELECT * FROM reference_rates;
CREATE TABLE reference_rates_new (id BIGSERIAL PRIMARY KEY, data_ts TIMESTAMPTZ NOT NULL);
DROP TABLE reference_rates CASCADE;
ALTER TABLE reference_rates_new RENAME TO reference_rates;
CREATE VIEW vw_reference_rates AS SELECT * FROM reference_rates;

• Before: sqlc generate → relation "vw_reference_rates" already exists (exit 1)
• After: exit 0

Fix

• internal/sql/ast: carry the DROP behavior (RESTRICT/CASCADE) into DropTableStmt, and add named DropBehavior constants matching the pg_query enum (UNDEFINED=0, RESTRICT=1, CASCADE=2).
• internal/engine/postgresql/parse.go: thread n.Behavior into the constructed DropTableStmt.
• internal/sql/catalog: record each view's referenced tables (DependsOn) at createView time by walking the view query for *ast.RangeVar nodes; on DROP TABLE ... CASCADE, evict dependent views transitively (a view may depend on another view).
• RESTRICT and unspecified behavior are unchanged — dependent views are kept, preserving today's behavior.

Tests

Added catalog unit tests covering:

• DROP TABLE ... CASCADE evicts a dependent view, including views-on-views (transitive).
• DROP TABLE ... RESTRICT and bare DROP TABLE keep the dependent view and preserve its recorded dependency.

go test ./internal/sql/catalog/... ./internal/engine/postgresql/... passes; gofmt/go vet clean.