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>

routers/api/v1/repo/pull.go

@@ -657,6 +657,15 @@ func EditPullRequest(ctx *context.APIContext) {
 		return
 	}
 
+	// Fail fast: if content_version is provided and already stale, reject
+	// before any mutations. The DB-level check in ChangeContent still
+	// handles concurrent requests.
+	// TODO: wrap all mutations in a transaction to fully prevent partial writes.
+	if form.ContentVersion != nil && *form.ContentVersion != issue.ContentVersion {
+		ctx.APIError(http.StatusConflict, issues_model.ErrIssueAlreadyChanged)
+		return
+	}
+
 	if len(form.Title) > 0 {
 		err = issue_service.ChangeTitle(ctx, issue, ctx.Doer, form.Title)
 		if err != nil {
@@ -665,10 +674,14 @@ func EditPullRequest(ctx *context.APIContext) {
 		}
 	}
 	if form.Body != nil {
-		err = issue_service.ChangeContent(ctx, issue, ctx.Doer, *form.Body, issue.ContentVersion)
+		contentVersion := issue.ContentVersion
+		if form.ContentVersion != nil {
+			contentVersion = *form.ContentVersion
+		}
+		err = issue_service.ChangeContent(ctx, issue, ctx.Doer, *form.Body, contentVersion)
 		if err != nil {
 			if errors.Is(err, issues_model.ErrIssueAlreadyChanged) {
-				ctx.APIError(http.StatusBadRequest, err)
+				ctx.APIError(http.StatusConflict, err)
 				return
 			}