fix(charm): check image readiness before starting runner manager service

[cbartz]

Overview

Moved _check_image_ready() into _reconcile() so every code path that starts the runner manager service first verifies the image integration has an available image.

Rationale

Several event handlers (_on_upgrade_charm, _on_planner_relation_changed, _on_planner_relation_broken) called _reconcile() without checking image readiness. This caused the service to start with combinations: [] and error with "No runner
combinations configured" instead of setting the charm to blocked/waiting status.

Juju Events Changes

No new events. Existing handlers _on_upgrade_charm, _on_planner_relation_changed, _on_planner_relation_broken, _on_config_changed, _on_image_relation_changed, and _on_debug_ssh_relation_changed now all go through _reconcile() for image readiness
checks. Redundant _check_image_ready() calls removed from the latter three.

Module Changes

• charm.py: _reconcile() now calls _check_image_ready() before _setup_service().

Library Changes

None.

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.
• The application version number is updated in github-runner-manager/pyproject.toml.