Feat/get configuration action

[minulo]

What this PR does

Add a get-configuration Juju action so operators can inspect the effective haproxy configuration for debugging without shell access to the unit.

• source=disk (default): return the applied /etc/haproxy/haproxy.cfg
• source=relations: preview the configuration the next reconcile would generate from the current haproxy-route relation data (read-only; no file writes or service reload)
• Log a warning when the effective configuration matches the default (no proxy backends configured) and, in relations mode, when a haproxy-route-policy relation makes the policy backend converge asynchronously (source=disk may also face the async issue if run before the haproxy-route-policy get written to file)
• Refactored shared template-context and render-to-string helpers so the reconcile and preview paths cannot drift

Why we need it

The haproxy charm creates configuration based on its haproxy-route, haproxy-route-policy, and haproxy-route-tcp relations, but there is no way to inspect the effective configuration without shell access to the unit. This spec proposes a get-configuration Juju Action that returns the charm’s configuration out to stdout by both reading from disk and calculating the potential resulting configuration based on current relations.

Checklist

• [ x] I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• [ x] I used AI to assist with preparing this PR
• [ x] I added or updated tests as needed (unit and integration)