fix(sqlite): preserve NOT NULL from schema when explain loses it through ORDER BY

[barry3406]

Fixes #4147

When a query includes ORDER BY, SQLite routes data through an ephemeral sorter table. This causes two problems for nullability inference:

1. The EXPLAIN-based analysis loses NOT NULL constraints as data passes through the ephemeral table
2. sqlite3_column_table_name() returns NULL for columns from ephemeral tables, so the schema-based column_nullable() fallback also returns None

Before:

// exp_nullable = Some(true) (explain lost NOT NULL through sorter)
// col_nullable = None (origin unknown for ephemeral column)
nullable.push(exp_nullable.or(col_nullable));
// Result: Some(true) — incorrectly says nullable

The macro then generates Option<String> instead of String, causing a compile error.

After:
If the schema definitively says NOT NULL (col_nullable = Some(false)), that takes priority over explain. The schema is the source of truth for column constraints.

// col_nullable says NOT NULL → trust it
(Some(false), _) => Some(false)

This fixes the case where query_as! considers TEXT NOT NULL as nullable when ORDER BY is present.

[barry3406]

Closing this — my fix was too aggressive and breaks LEFT JOIN / UNION nullability inference. The real issue is in the explain analyzer losing NOT NULL through ORDER BY + LIMIT ephemeral tables. Need to dig deeper into the EXPLAIN opcode tracking. Sorry for the noise.