This PR adds some support for diff / merge in the remote client as for local tables we stay NotSupported until lance-format/lance#7263.

This wires the two review-and-land calls against the remote REST API:

• POST /v1/table/{id}/branches/diff
• POST /v1/table/{id}/branches/merge

Rust gets typed results (BranchDiff, MergeBranchResult). Python returns the wire JSON, same shape as the REST response.

Merge here means promoting a branch's added columns onto main.

Behavior

• Remote only. Local raises NotSupported.
• A rejected merge is not an exception. HTTP 409 still returns Ok / a dict with status="rejected" and blockers in diff.mergeBlockers.
• Unknown blocker / status codes parse as Unknown so a newer server does not break older clients.
• MergePreview tolerates missing fields for the same reason.
• Merge requests are not retried. 409 is final and carries the body you need.

Example

table = db.open_table("images")

table.branches.create("exp")
exp = table.branches.checkout("exp")

exp.add_columns({"tag": "cast('draft' as string)"})

diff = table.branches.diff("exp")
preview = table.branches.merge("exp", dry_run=True)
result = table.branches.merge("exp", dry_run=False)

if result["status"] == "merged":
    print("landed at", result["mainVersionAfter"])
elif result["status"] == "rejected":
    print(result["diff"]["mergeBlockers"])

Testing

cargo test -p lancedb --features remote diff_branch
cargo test -p lancedb --features remote merge_branch