Report configured reclamation window in provider status responses

[linear]

Why

Exposing the field in the API is not enough — the running provider must report its configured value so inventory and bid-screening clients can rely on it.

What

Provider status responses report the provider's configured reclamation window. Providers without a configured window omit the field.

Flows that should work:

1. REST /status includes reclamation_window when configured.
2. gRPC GetStatus and StreamStatus include Status.reclamation_window.

Acceptance Criteria

• /status includes reclamation_window when configured, and omits it when not.
• gRPC GetStatus and StreamStatus include Status.reclamation_window matching the configured --reclamation-window.
• Existing bidengine behavior is unchanged.
• Tests cover provider REST/gRPC status wiring.