feat(rest): support atomic multi-table commits

[YuangGao]

Which issue does this PR close?

• Closes REST catalog: client support for atomic multi-table commits (POST /v1/{prefix}/transactions/commit) #2784.

What changes are included in this PR?

Adds client support for POST /v1/{prefix}/transactions/commit, which commits changes to several tables atomically. Continues #2786 by @steveis

• Transaction::prepare_commit() builds a transaction's TableCommit without refreshing or sending it; do_commit now uses it too.
• RestCatalog::commit_transaction / RestSessionCatalog::commit_transaction send the group in one request. The server returns no metadata, so callers reload the tables; on a conflict nothing is applied.

cc @CTTY: prepare_commit takes &mut self instead of consuming the transaction, so it can still report the commit status for the cleanup planned in RFC-0003. Does that match what you have in mind?

Are these changes tested?

• Unit tests for prepare_commit and, with mockito, the request body, 409, an unadvertised endpoint, and an empty group.
• Integration tests against the REST fixture: two tables commit together, and a stale requirement on one table fails the group with 409 while the other table stays unchanged.

AI Disclosure

• AI-assisted implementation.