Handle session creation failures

A session that reaches CREATING has no failure path. No lifecycle handler targets SessionStatus.CREATING, and the CHECK_CREATING_PROGRESS promotion spec only produces successes, so SessionHandlerOptions timeout / max_retry_count are never consulted. When kernel creation fails on the agent side without emitting a termination event, the session stays in CREATING forever and the user sees no error.

Scope: detect and surface creation failures so a session leaves CREATING with a meaningful status and reason.

- Propagate agent-side kernel creation errors to the manager.
- Apply a phase timeout / retry budget to the CREATING phase.
- Expose the failure reason to the user instead of an indefinite CREATING state.

JIRA Issue: BA-7725

Start at the SessionStatus.CREATING lifecycle handling and the CHECK_CREATING_PROGRESS promotion spec, then trace SessionHandlerOptions timeout and max_retry_count between the agent and manager. Use the promotion spec and related lifecycle tests to verify that failed creation exits CREATING with a user-visible status and reason.

Written by the indexing model from the issue text.