500 instead of 409 when deactivating an already-deactivated or not-yet-published DID

Split from #1883 (part 2).

## Description

`POST /did-registrar/dids/{didRef}/deactivations` returns `500` ("An unexpected error occurred when servicing the request") when it should return `409 Conflict` with a proper error body. Affected cases:

1. Deactivating an already-deactivated DID (`DIDAlreadyDeactivated`)
2. Deactivating a DID that is `CREATED` or `PUBLICATION_PENDING` (`DIDNotPublished`)
3. The `MultipleInflightUpdateNotAllowed` path (same mapping)

## Root cause

- The service layer correctly raises `UpdateManagedDIDError.DIDAlreadyDeactivated` / `DIDNotPublished` (`ManagedDIDServiceImpl.deactivateManagedDID`).
- The controller maps both to `ErrorResponse.conflict` (409) (`DIDRegistrarController`).
- But the `deactivateManagedDid` endpoint declares `errorOut(EndpointOutputs.basicFailuresAndForbiddenWith(FailureVariant.notFound))`, i.e. only 400/401/403/404/422/500. Unlike the sibling `updateManagedDid` endpoint, it has no `FailureVariant.conflict`.
- At runtime the 409 response fails tapir's `oneOf` encoding (no variant matches), the defect falls through to the generic handler in `CustomServerInterceptors`, and surfaces as 500.

Same class of bug as #1537 (fixed narrowly for the 401 variant on `/connections` in #1540).

## Fix

- Add `FailureVariant.conflict` to the `deactivations` endpoint's `errorOut`.
- Regenerate the OpenAPI spec.
- Add an HTTP-level test for the double-deactivation path (current tests only assert the service-layer error, so the missing variant was never caught).

## Environment

Observed on identus-cloud-agent 2.2.0, reproducible on current `main` (`85cb017f`).

Start with the deactivateManagedDid endpoint's errorOut declaration and compare it with the sibling updateManagedDid endpoint, then review DIDRegistrarController and the existing service-layer tests. Regenerate the OpenAPI spec and add an HTTP-level test for double deactivation; done means the affected cases return 409 Conflict with the proper error body instead of 500.

Written by the indexing model from the issue text.