Validate unknown columns in SELECT WHERE clauses (fixes #4264)

[lawrence3699]

sqlc currently validates bad column references in SELECT targets, GROUP BY, and ORDER BY, but it lets unknown columns in SELECT WHERE clauses slip through to generated code. That means queries like select * from "user" where is_deleted = false pass sqlc generate even though PostgreSQL rejects them at execution time. Fixes #4264.

Run column-reference validation over SELECT WHERE clauses as well, and recurse into nested SELECTs so the same check also catches unknown columns inside EXISTS/scalar subqueries without breaking correlated references to outer scopes.

Add end-to-end fixtures for both the flat WHERE case and a nested EXISTS subquery case, with base and managed-db expectations.

Validation:

• go test ./internal/endtoend -run 'TestReplay/base/invalid_where_unknown_column/postgresql/pgx$' -count=1
• go test ./internal/endtoend -run 'TestReplay/base/invalid_where_unknown_column_subquery/postgresql/pgx$' -count=1
• POSTGRESQL_SERVER_URI='postgresql://lawrence@/postgres?host=/tmp&sslmode=disable' go test ./internal/endtoend -run 'TestReplay/managed-db/invalid_where_unknown_column/postgresql/pgx$' -count=1
• POSTGRESQL_SERVER_URI='postgresql://lawrence@/postgres?host=/tmp&sslmode=disable' go test ./internal/endtoend -run 'TestReplay/managed-db/invalid_where_unknown_column_subquery/postgresql/pgx$' -count=1
• go test ./internal/endtoend -run 'TestReplay/base/select_exists/pgx/v5$' -count=1
• go test ./internal/endtoend -run 'TestReplay/base/select_subquery/postgresql/stdlib$' -count=1