test(garm): refactor unit tests to the scenario framework

AGENTS.md

@@ -72,6 +72,22 @@ For **`garm-configurator`** (plain `ops`):
 
 - New unit tests use **Scenario** (`scenario.Context` / `State` / `Relation` / `Secret`),
   not `Harness` — see `charms/garm-configurator/tests/unit/`.
+- **DO** give every Python test an arrange/act/assert docstring — the convention `STYLE.md`
+  documents for Go (`// Arrange:` comments), spelled as a docstring:
+  ```python
+  def test_leader_creates_garm_secrets():
+      """
+      arrange: A leader unit with no GARM secrets yet.
+      act: Run install.
+      assert: Both labelled secrets are created, so other units can fetch them by label.
+      """
+  ```
+  State the *why* in the assert line where it isn't obvious ("so other units can fetch them
+  by label"), not just the mechanical result. For a `parametrize`d test, write one docstring
+  covering the cases — **DON'T** write one per case.
+- **DO** fix a missing AAA docstring on any test you move or edit. It's followed unevenly
+  (`planner-operator` and `garm` yes; `garm-configurator` and `webhook-gateway-operator` not
+  yet), so imitating the nearest neighbour is not a reliable guide.
 - Integration tests live in the shared `charms/tests/integration/`.
 
 ## 12-factor divergences from the canonical charm-engineer guidance

charms/garm/AGENTS.md

@@ -35,3 +35,11 @@ charm conventions; this file lists only what's specific to `garm`.
   rather than blocking). The port is pinned in the `_workload_config` property.
 - Tests: unit in `tests/unit/`; integration via `tox -e garm-integration`
   (`charms/tests/integration/test_garm.py`).
+- **Scenario tests** — `ops-scenario` expands the `go-framework` extension itself, so this charm
+  is testable from source with no packed charm. Reaching `ActiveStatus` needs a few fixtures a
+  plain `ops` charm doesn't; the state builder in `tests/unit/test_charm.py` assembles them, each
+  annotated with the `paas_charm` behaviour that demands it. **DO** start from it rather than
+  building a `State` from scratch.
+  - **DON'T** rebuild the mock-`self` pattern (`GarmCharm.restart(MagicMock())`) these tests
+    replaced: it can't see what the charm pushed or replanned, so it hid real bugs. Stub only
+    GARM's HTTP surface (the `garm_api` fixture) and assert on the output `State`.