Wrap errors in GetPullRequest and PostReview for consistency

Add fmt.Errorf wrapping to the two remaining unwrapped error returns
in the adapter:
- GetPullRequest: 'get pull request: %w'
- PostReview (final client call): 'post review: %w'

This makes all error paths in the adapter consistent with the wrapping
pattern used by the diff-fetch and position-translation errors.

Addresses self-review findings #1 and #2 from b2eea502.

gitea/adapter.go

@@ -34,7 +34,7 @@ func (a *Adapter) Underlying() *Client {
 func (a *Adapter) GetPullRequest(ctx context.Context, owner, repo string, number int) (*vcs.PullRequest, error) {
 	pr, err := a.client.GetPullRequest(ctx, owner, repo, number)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("get pull request: %w", err)
 	}
 	return &vcs.PullRequest{
 		Number: number,
@@ -180,7 +180,7 @@ func (a *Adapter) PostReview(ctx context.Context, owner, repo string, number int
 
 	review, err := a.client.PostReview(ctx, owner, repo, number, event, req.Body, giteaComments)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("post review: %w", err)
 	}
 
 	return &vcs.Review{