fix(batch-changes): disallow retry on deleted changesets (#63336)

Closes SRCH-524

[Slack
Context](https://sourcegraph.slack.com/archives/C05EA9KQUTA/p1718230808589239?thread_ts=1718230808.589239&cid=C05EA9KQUTA)

Bitbucket Server (v5.1) added the ability to decline and [delete pull
requests](https://confluence.atlassian.com/bitbucketserver/bitbucket-server-5-1-release-notes-902140366.html)
- this isn't replicated across other supported codehosts such as:

- GITHUB
- GITLAB
- BITBUCKET CLOUD
e.t.c


https://github.com/sourcegraph/sourcegraph/assets/25608335/e4e448ee-36a5-4b5a-8265-b47a9ae88628

Due to this, whenever a pull request is deleted, Batch Changes services
(like Syncer & Reconciler) keep retrying whatever operation they're
running, thereby spamming the codehost in the process.
In this PR, I ensured whenever we encounter certain status code from a
HTTP request to a codehost, it returns a NonRetryable error to reduce
spamming.

The logic for handling non-existent pull requests [lived
here](https://sourcegraph.sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/extsvc/bitbucketserver/client.go?L464-467),
however that condition was never met because `httpError` did not fulfil
the `Unwrap` interface therefore `errors.As` couldn't unwrap the
underlying error.


## Test plan

Create a test batch change targeting a bitbucket server codehost. Once
the pull request has been created, delete the pull request.

Once the changeset has been synced on Sourcegraph, the status of the
changeset will be updated to`CLOSED`.
```yaml
name: testbbdecline2
description: Add Hello World to READMEs

# Find the first 100 search results for README files.
# This could yield less than 100 repos/workspaces if some repos contain multiple READMEs.
on:
  - repositoriesMatchingQuery: repo:^bitbucket\.sgdev\.org/BABU/(delve|compose)$ 

# In each repository, run this command. Each repository's resulting diff is captured.
steps:
  - run: IFS=$'\n'; echo Hello World | tee -a $(find -name README.md)
    container: ubuntu:18.04

# Describe the changeset (e.g., GitHub pull request) you want for each repository.
changesetTemplate:
  title: Hello World
  body: My first batch change!
  commit:
    message: Append Hello World to all README.md files
  # Optional: Push the commit to a branch named after this batch change by default.
  branch: ${{ batch_change.name }}
```

## Changelog

* Disallow auto-retry when a changeset is deleted.

Author: BolajiOlajide

internal/errcode/code.go

@@ -188,6 +188,8 @@ type nonRetryableError struct{ error }
 
 func (nonRetryableError) NonRetryable() bool { return true }
 
+func (e nonRetryableError) Unwrap() error { return e.error }
+
 func MaybeMakeNonRetryable(statusCode int, err error) error {
 	if statusCode > 0 && statusCode < 200 ||
 		statusCode >= 300 && statusCode < 500 ||

internal/extsvc/bitbucketserver/client.go

@@ -1011,12 +1011,12 @@ func (c *Client) do(ctx context.Context, req *http.Request, result any) (_ *http
 		return resp, err
 	}
 
-	if resp.StatusCode < 200 || resp.StatusCode >= 400 {
-		return resp, errors.WithStack(&httpError{
+	if resp.StatusCode < http.StatusOK || resp.StatusCode >= http.StatusBadRequest {
+		return resp, errors.WithStack(errcode.MaybeMakeNonRetryable(resp.StatusCode, &httpError{
 			URL:        req.URL,
 			StatusCode: resp.StatusCode,
 			Body:       bs,
-		})
+		}))
 	}
 
 	// handle binary response

internal/extsvc/bitbucketserver/client_test.go

@@ -441,6 +441,12 @@ func TestClient_LoadPullRequest(t *testing.T) {
 				t.Fatalf("error:\nhave: %q\nwant: %q", have, want)
 			}
 
+			if tc.name == "non existing pr" {
+				if err != ErrPullRequestNotFound {
+					t.Fatal("expected error, got nil")
+				}
+			}
+
 			if err != nil || tc.err != "<nil>" {
 				return
 			}