remove_backend on a static backend doubles the conflict: prefix #306

Problem

remove_backend on a statically configured backend answers conflict: conflict: backend ollama is configured statically …: the error prefix is doubled, because the conflict error is wrapped by a caller that adds the same prefix again. Seen with 1.16.1-rc.2 in a local agentlab.

Proposed solution

Wrap the conflict sentinel once: the inner error carries the message, the outer layer only classifies it (errors.Is) without prefixing it again.

Acceptance criteria

• remove_backend on a static backend answers conflict: backend <name> is configured statically … with one prefix.
• A test asserts the exact message.