Support the `If-None-Match` header

Expected behaviour

• POST a resource to an IRI where a resource already exists, with the If-None-Match header set.
• The server returns 412, and the original resource is unchanged.

Actual behaviour

• POST a resource to an IRI where a resource already exists, with the If-None-Match header set.
• The server returns 201 Created, and the original resource is overwritten.

Additional context

When receiving a resource creation request (i.e. POST or PUT, possibly PATCH?), if the If-None-Match header is set, the server should perform a check to ensure that no resource is already stored at the target IRI.

In the case of POST, since the Slug header is meant to be a hint and not a hard constraint, I'm not sure what the behaviour should be, but when PUTting a resource to an IRI where a resource already exists, an If-None-Match request should fail with 412.

Open question

When only having write (but not read) access to a container, can this be used to exfiltrate information one should not be allowed to access ?

No files, tests, or entry points are named. Start by locating the server's POST and PUT request handling and its conditional-header processing, then review the existing behavior around If-None-Match. Done means the requested 412/no-overwrite behavior is covered, with the POST/Slug and write-only access questions resolved.