Add optimistic locking conflict test

[fleuronvilik]

Objective

Validate version-based concurrency behavior in PartnerInventory updates.

Scope

• Add a conflict test where two writes contend on the same version.
• Assert typed error and consistent final state.

Acceptance Criteria

• Conflict scenario is reproducible in tests.
• Failure is explicit and typed (no generic Exception/ValueError).
• No silent overwrite occurs.

Why

Concurrency safety is a core backend skill and prevents subtle data corruption.