* fix(server): validate assignee_id existence on issue create/update
POST /api/issues and PUT /api/issues/:id silently accepted any
well-formed UUID as assignee_id (#1662). The new validateAssigneePair
helper consolidates the existing canAssignAgent check and adds:
- existence lookup against workspace members for assignee_type=member
- existence lookup against workspace agents for assignee_type=agent
- pair consistency: type and id must be both set or both null
- whitelist for assignee_type values (member|agent)
UpdateIssue and BatchUpdateIssues now run the same validator on the
post-merge assignee pair whenever the caller touches either field,
closing the parallel gap on the update path.
* fix(server): reject malformed assignee_id at handler entry
parseUUID silently returns an invalid pgtype.UUID for unparseable input
and validateAssigneePair treats (type unset + id invalid) as "no
assignee". Together they let `POST /api/issues` and `PUT /api/issues/:id`
silently drop a malformed assignee_id and return a successful response.
Reject the parse failure inline at every entry point — Create, Update,
and BatchUpdateIssues — so the validator never sees an unparseable id.
Adds two regression tests covering the create and update paths.