Add assignees APIs

[lunny]

Follow https://docs.github.com/en/enterprise-server@3.20/rest/issues/assignees?apiVersion=2022-11-28
Fix #33576

[Copilot]

Pull request overview

This PR adds GitHub-compatible REST endpoints for checking, adding, and removing issue assignees, and wires them into routing, swagger, and integration tests. It also includes some related refactoring in assignee/review-request service code and a small model signature change.

Changes:

• Add new API endpoints: repo-level “check assignee” and issue-level add/remove/check assignees.
• Extend swagger (templated spec + parameter bodies) and add integration coverage for the new endpoints.
• Refactor/move assignee + review-request helpers and update IsUserAssignedToIssue to take a userID instead of a *User.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/integration/api_repo_test.go	Adds integration coverage for GET /repos/{owner}/{repo}/assignees/{assignee}.
tests/integration/api_issue_test.go	Adds integration coverage for add/remove/check issue assignees endpoints.
templates/swagger/v1_json.tmpl	Documents the new assignee endpoints + IssueAssigneesOption.
routers/api/v1/swagger/options.go	Registers IssueAssigneesOption as a swagger body type.
routers/api/v1/api.go	Routes the new repo/issue assignee endpoints.
routers/api/v1/repo/collaborators.go	Implements repo-level assignee eligibility check endpoint.
routers/api/v1/repo/issue.go	Implements issue-level add/remove/check assignees endpoints.
modules/structs/issue.go	Adds IssueAssigneesOption.
models/repo/user_repo.go	Adds IsRepoAssignee helper mirroring GetRepoAssignees eligibility rules.
models/issues/assignees.go	Changes IsUserAssignedToIssue signature; simplifies MakeIDsFromAPIAssigneesToAdd return.
models/issues/assignees_test.go	Updates tests to the new IsUserAssignedToIssue signature.
services/context/repo.go	Updates call site for IsUserAssignedToIssue signature change.
services/issue/issue.go	Moves assignee helpers out of this file.
services/issue/assignee.go	Adds bulk add/remove helpers; moves assignee helpers here.
services/issue/assignee_test.go	Updates test for IsUserAssignedToIssue signature change.
services/issue/review_request.go	Moves review-request logic into a dedicated file (refactor).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.