jira issue assign fails with '400 Bad Request: No project, issue key or issue ID was provided'

## Description

`jira issue assign` consistently fails with a `400 Bad Request` error regardless of project or user format.

## Steps to Reproduce

```bash
jira issue assign DEV-207949 "user@domain.com"
jira issue assign ITP-41459 "other.user@domain.com"
jira issue assign ITP-41459 username
```

All produce the same error:

```
Error:
- No project, issue key or issue ID was provided

jira: Received unexpected response '400 Bad Request'.
Please check the parameters you supplied and try again.
```

## Expected Behavior

The issue should be assigned to the specified user, matching the documented usage:

```bash
jira issue assign ISSUE-1 jon@domain.tld
```

## Environment

- **Version**: Built from source (fork), base `v1.7.1`
- **Go**: `go1.25.6`
- **Platform**: `darwin/arm64`
- **Jira Cloud**: Atlassian-hosted instance

## Workaround

Using the Jira REST API directly works:

```bash
curl -s -u "user@domain.com:$TOKEN" \
-X PUT "https://instance.atlassian.net/rest/api/3/issue/ISSUE-KEY/assignee" \
-H "Content-Type: application/json" \
-d '{"accountId": ""}'
```

## Notes

- The error message `"No project, issue key or issue ID was provided"` comes from the Jira API, suggesting the CLI is not sending the issue key correctly in the request body.
- Other commands like `jira issue view`, `jira issue comment add`, and `jira issue edit` work fine with the same issue keys.
- Possibly a regression in how the assign command constructs the API request payload.