fix(sqlite): correctly infer nullability for INTEGER PRIMARY KEY in RETURNING

[at264939-ctrl]

fixes #4322. This is a fix for a regression introduced in 0.9.0.

After upgrading to sqlx 0.9.0, many users noticed that INSERT ... RETURNING queries on INTEGER PRIMARY KEY columns started returning Option instead of T, which broke a lot of code that expects a simple i64. I traced this down to commit 69ee0df from PR #4088.

The issue is that in sqlx-sqlite/src/statement/handle.rs, column_nullable was changed to return None for rowid aliases. This forces the logic to fall back to the bytecode simulation in explain.rs, which relies on PRAGMA table_info. In SQLite, this pragma returns notnull=0 for INTEGER PRIMARY KEY columns because they can be passed as NULL during an insert. However, in any actual result set, these columns are never NULL.

To fix this, I updated root_block_columns in explain.rs to include the pk column from table_info so the VM simulation can properly identify these rowid aliases and mark them as non-nullable. I also updated handle.rs to return Some(false) instead of None for these cases to provide a more accurate initial inference. Finally, I adjusted a few existing tests in explain.rs that were previously asserting these columns as nullable. I've verified the fix with integration tests and it correctly restores the expected i64 behavior

[at264939-ctrl]

Is there no one here for review?