[RFE] - add HA test suite for sharding

## Steps to reproduce

1.

## Expected behavior

## Actual behavior

## Versions

Operating system:

Juju CLI:

Juju agent:

Charm revision:

LXD:

## Log output

Juju debug log:

## Additional context

The issue provides no files, tests, entry points, or acceptance criteria. Start by locating the existing sharding and high-availability test coverage in the repository, then define the scenarios and environments the suite must cover before implementation. Done should mean the agreed HA sharding tests run successfully and are documented.

Written by the indexing model from the issue text.