fix(federation): relay FM's 409 when a partner already has an agreement

FM now keeps one agreement per partner, edited in place: a second
POST /internal/agreements answers 409 agreement-exists, and so does a
PATCH that would bring an expired agreement back next to a live one.
The relay's FM contract listed no 409 for either route, so the operator
got a 502 instead of FM's ProblemDetails telling them which agreement to
edit. Add 409 to both, in the status allowlists and the OpenAPI
responses, and reword the PATCH summary now that any term can change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>