Add and remove a run's tags

Adds the tags in add and removes the tags in remove, then returns the run's full tag list, sorted. Both are set operations: adding a tag the run already has, or removing one it does not have, changes nothing, so a repeated request is safe without an idempotency key, and two callers editing one run do not erase each other's tags. A tag in both lists is ambiguous and returns 400. Requires run:write.

A run has at most 20 tags. The limit applies to the result, so a request may add tags as it removes others. A result over 20 tags returns 400 naming add and changes nothing. A tag is 1 to 64 characters, stored exactly as sent: it is case-sensitive, nothing is trimmed, and a tag that is empty, only whitespace or contains a control character returns 400. A tag is any other text, so choose the convention you like.

Changing tags never changes the run's turns, their inputs and outputs, the deployment version they ran on, or the work in progress. It does not keep an idle run retained longer.