Verify and enforce NOT NULL on sessions / kernels identity columns

Session/kernel identity columns are always populated on the creation path but are declared nullable. Verify there is no scheduling-intermediate state or legacy data that legitimately leaves them NULL, then migrate to NOT NULL.

Target columns:

- sessions.name, sessions.access_key, sessions.creation_id
- kernels.session_name, kernels.access_key, kernels.image, kernels.session_creation_id

For each column: backfill existing NULL rows, ensure an appropriate default/server default, add an Alembic migration setting NOT NULL, and update the ORM model type annotations.

JIRA Issue: BA-7220

Start by tracing the sessions and kernels creation paths and checking for scheduling-intermediate states or legacy rows with NULL identity fields. Review the ORM models and Alembic migration history, then verify that existing NULLs can be backfilled and that the migration, defaults, NOT NULL constraints, and type annotations cover all listed columns.

Written by the indexing model from the issue text.