Reuse existing JSON groups

[johngodley]

When importing JSON, re-use any group if it already exists

[Copilot]

Pull request overview

Updates the JSON import path to re-use existing redirect groups when an incoming JSON group references an ID that already exists in the database, instead of always creating a new group. This aligns JSON imports with real-world scenarios where exports/imports may be repeated into the same site.

Changes:

• Refactors JSON import group mapping to resolve group IDs via a new get_group_id() helper that prefers existing groups.
• Ensures missing group IDs referenced by redirects create a single fallback “Group” per distinct missing ID (via mapping reuse).
• Adds integration tests covering “reuse existing group ID” and “single fallback group per missing ID”.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/integration/fileio/test-json.php	Adds integration coverage for reusing existing groups and for fallback-group behavior when group IDs are missing.
includes/fileio/format/class-json.php	Refactors JSON import to resolve group IDs through get_group_id() and re-use existing groups when present.

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