test: driver deadline call sites

[meelodev]

Background

Driver deadline behavior is enforced in several call sites, not in one central path.

Before adding broader cancellation logic, it would help to lock down the existing timeout and expired-deadline behavior.

Details

Add focused tests for current driver deadline call sites. No runtime behavior change.

Acceptance criteria

• Test that an expired solver deadline does not send a /solve request.
• Test post-processing timeout behavior where practical.
• Test that settlement rejects when the submission deadline has passed.
• Changes are limited to tests and small test helpers.
• No new production dependencies.