fix(server): Return 200 instead of 204 for object HEAD requests

[jan-auer]

A HEAD on an existing object now responds with 200 OK and the metadata headers instead of 204 No Content. Callers that branch on the status code no longer have to special-case HEAD against the 200 that GET returns for the same resource.

Additionally, GET responses now return the content-length instead of streaming with chunked transfer encoding.

This aligns with RFC 9110 §9.3.2, which specifies that a HEAD response should carry the same status and header fields as the equivalent GET, minus the body, and with what both S3 HeadObject and the GCS XML API do. It also unblocks exposing the object size on HEAD later: 204 responses are forbidden from carrying Content-Length, so the old status permanently foreclosed the cheapest way for a client to learn an object's size.

Also fixes the S3-compatible backend reporting a size of zero for metadata-only lookups, since it read the decoded body length rather than the Content-Length header.

[cursor]

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[lcian]

Looks good to me.