Fix: Ken drops databases on nodedown

[Harshit-jain-57]

Description

This PR fixes an issue where Ken (the background indexer) would drop a database from its indexing queue if fabric:design_docs/1 returned a nodedown or maintenance_mode error.

Previously, these errors were caught and treated as an empty list of design documents ({ok, []}), causing Ken to believe there was nothing to index and removing the database from the pending queue.

Changes

• Modified ken_server:design_docs/1 to propagate {error, {nodedown, ...}} and {error, {maintenance_mode, ...}} instead of swallowing them.
• Modified ken_server:update_db_indexes/2 to handle these specific errors by exiting with resubmit. This triggers Ken's built-in retry mechanism, ensuring the database is added back to the queue to be retried later.

Testing

• Added a reproduction test case in src/ken/test/ken_repro_test.erl (if applicable to include in PR).
• Verified that nodedown errors now trigger a resubmit instead of a silent drop.

Fixes #5392<!-- Thank you for your contribution!

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Overview

Testing recommendations

Related Issues or Pull Requests

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches