Truncate Jira issue keys at the 500 per-entity API limit

What

Jira Cloud rejects a build, a dev-info entity (commit, branch, or merge request), or a feature flag that carries more than 500 issue keys, with errors such as 'issueKeys' can have a maximum of 500 elements and Too many issue keys in entity. The affected data is then not linked in the Jira development panel.

gitlab-org/gitlab!231117 fixed this for deployments by truncating to 500. This MR applies the same handling to the builds, dev-info, and feature-flag bulk endpoints.

How

A shared truncate_issue_keys helper caps issueKeys at 500 per entity and reports the dropped count to Sentry, tagged by endpoint. Feature-flag keys cannot exceed 500 in practice, so that cap is defensive. Deployment handling is unchanged.

Related to gitlab-org/gitlab#470092