REST: Send If-None-Match on table refresh

[raunaqmorarka]

RESTTableOperations.refresh() always issued an unconditional GET and parsed the full metadata body, even though the same catalog already does freshness-aware loading for loadTable through RESTTableCache. Servers that honour If-None-Match (for example Polaris) answer with 304 Not Modified, so a refresh of an unchanged table only needs a header round trip.

RESTTableOperations now keeps the ETag from the load response that created it and from every successful refresh and commit response, sends it as If-None-Match on refresh(), and returns current() on a 304 without parsing a body. When the server sends no ETag, refresh behaves as before. The RESTTableCache entry is left untouched, since the ETag there pairs with the metadata captured by the cached supplier.

Note: commit() now uses the RESTClient.post overload that returns response headers. BaseHTTPClient supports it; a custom RESTClient relying only on the interface default would need to implement it.

Tests cover a conditional refresh returning 304, a changed table still refreshing fully and adopting the new ETag, a commit ETag being reused, servers that send no ETag, and the 304 path over the Jetty-backed test server.