refactor(garm): make teardown the shared cleanup coordinator

[yanksyoon]

Summary

This child PR is stacked on #333 and refactors the existing removal cleanup into the shared _teardown() orchestrator.

• Rename the removal observer target from _on_remove() to _teardown().
• Use the same orchestrator for local GARM teardown routed by _reconcile(), PostgreSQL relation-departed, stop, and remove.
• Keep cleanup independent of PostgreSQL relation data and event application/unit fields.
• Keep active behavior unchanged when planned_units() > 0.

Relation-independent teardown

_teardown() uses only:

• local planned_units() and leadership;
• the application-owned GARM admin secret;
• the local GARM API; and
• the existing GarmResourceCleanup API coordinator.

The old fallback that called _get_postgresql_config() when GARM was unreachable has been removed. Late teardown now logs an unconfirmed result and returns instead of reading a relation that may already be broken.

Pre-break relation-departed failures remain strict and raise GarmCleanupError so Juju can retry. Late stop, remove, and other teardown attempts tolerate unavailable API/database/credentials.

Idempotency

• _teardown_claimed prevents duplicate cleanup attempts from multiple observers in one hook process, notably the two update-status observers.
• A new hook process resets the claim and re-observes current GARM API state, so a killed hook can retry safely.
• GarmResourceCleanup disables resources before deletion, polls asynchronous runner removal, and treats already-missing resources as success.
• No force-removal or GitHub authorization bypass is enabled.

Validation

• tox -c tox.toml -e unit — 266 passed
• Focused teardown tests — 7 passed
• Ruff check and formatting — passed
• Complexity checks — passed
• Pyright — passed
• git diff --check — passed

Review focus

• _teardown() is intentionally global and leader-only, matching the existing _on_remove() behavior.
• The child PR does not add persistent-key migration or direct COS-helper lifecycle changes; those remain separate work.