Jira V2 contact point: search request sends 'expand' as empty string instead of omitting it

## Description

When using the Jira contact point with Jira Server/Data Center (REST API v2), the search for existing issues fails with a 400 error because the `expand` field in the `issueSearch` struct is serialized as an empty string (`"expand": ""`) instead of being omitted.

Jira Server v2 `/search` endpoint expects `expand` to be an array (`[]string`), not a string. Sending an empty string causes the following error:

```
failed to look up existing issues: HTTP request to JIRA API: webhook failed validation: unexpected status code 400:
{"errorMessages":["Can not deserialize instance of java.util.ArrayList out of VALUE_STRING token
at [Source: ...; line: 1, column: 2]
(through reference chain: com.atlassian.jira.rest.v2.search.SearchRequestBean[\"expand\"])"]}
```

## Root Cause

In `receivers/jira/v1/types.go`, the `issueSearch` struct defines `Expand` as:

```go
type issueSearch struct {
Expand string `json:"expand"`
Fields []string `json:"fields"`
JQL string `json:"jql"`
MaxResults int `json:"maxResults"`
}
```

Since `Expand` is a `string` without `omitempty`, and `getSearchJql()` in `jira.go` never sets the `Expand` field, it serializes to `"expand": ""` in the JSON body. Jira Server's v2 API rejects this because it expects an array.

## Suggested Fix

Add `omitempty` to the JSON tag:

```go
Expand string `json:"expand,omitempty"`
```

Or change the type to `[]string` to match Jira's expected format:

```go
Expand []string `json:"expand,omitempty"`
```

## Related

- PR #415 fixed the v2 endpoint path (`/search` instead of `/search/jql`) but did not address the `expand` field serialization.
- Issue https://github.com/grafana/grafana/issues/112921

## Environment

- **Grafana**: Cloud Advanced v12.4.0
- **Jira**: Server/Data Center (self-hosted, REST API v2)

## Steps to Reproduce

1. Configure a Jira contact point in Grafana Alerting with a Jira Server v2 URL (e.g., `https://your-jira.com/rest/api/2`)
2. Fill in Project Key, Issue Type, and authentication
3. Click "Test" to send a test notification
4. The test fails with the deserialization error above