Expose content_version for optimistic locking on issue and PR edits (#37035)

- Add `content_version` field to Issue and PullRequest API responses
- Accept optional `content_version` in `PATCH
/repos/{owner}/{repo}/issues/{index}` and `PATCH
/repos/{owner}/{repo}/pulls/{index}` — returns 409 Conflict when stale,
succeeds silently when omitted (backward compatible)
- Pre-check `content_version` before any mutations to prevent partial
writes (e.g. title updated but body rejected)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/structs/issue.go

@@ -80,7 +80,8 @@ type Issue struct {
 	PullRequest *PullRequestMeta `json:"pull_request"`
 	Repo        *RepositoryMeta  `json:"repository"`
 
-	PinOrder int `json:"pin_order"`
+	PinOrder       int `json:"pin_order"`
+	ContentVersion int `json:"content_version"`
 }
 
 // CreateIssueOption options to create one issue
@@ -114,6 +115,7 @@ type EditIssueOption struct {
 	// swagger:strfmt date-time
 	Deadline       *time.Time `json:"due_date"`
 	RemoveDeadline *bool      `json:"unset_due_date"`
+	ContentVersion *int       `json:"content_version"`
 }
 
 // EditDeadlineOption options for creating a deadline