Update contract metadata

Merges the keys you send into the contract's existing metadata, following JSON Merge Patch (RFC 7386) at the top level. A key with a value overwrites whatever is stored under that key; a key set to `null` removes it; a key you leave out of the payload is preserved. Removing a key that does not exist succeeds and changes nothing, so the same request can be replayed safely. Only top-level `null`s delete. The merge is shallow — sending an object as a value replaces the stored value outright rather than merging into it — so a `null` inside a nested object is stored as part of that object, not treated as a deletion. Removing every remaining key leaves an empty object, not null. Use DELETE on this path to clear the metadata back to null. Scoped to the authenticated merchant: a contract belonging to another merchant returns 404. The stored metadata is capped at 16KB, measured as the UTF-8 byte length of its JSON serialization. The cap applies to the merged result rather than to the request body, so a payload that is small on its own is still rejected with 400 if it would push the stored document over; the error names the attempted size. A request that only removes keys is always accepted, so metadata that is already over the cap can be brought back under it.