test: adopt -Werror in unit tests

docs/release-notes/artifacts/werror-unit-tests.yaml

@@ -0,0 +1,15 @@
+# --- Release notes artifact ----
+
+schema_version: 1
+changes:
+  - title: Run the unit tests with warnings promoted to errors
+    author: tonyandrewmeyer
+    type: chore
+    description: |
+      The unit tests now run with `filterwarnings = ["error", ...]`, so a deprecation or resource leak fails the suite instead of scrolling past in the log. Three third-party warnings are ignored by message; the charm's own code raises none. Integration tests are unaffected.
+    urls:
+      pr: https://github.com/canonical/indico-operator/pull/776
+      related_doc:
+      related_issue:
+    visibility: internal
+    highlight: false

pyproject.toml

@@ -17,6 +17,20 @@ pythonpath = [
     "lib",
     "src",
 ]
+# Turn warnings into errors so deprecations and resource leaks fail the unit
+# suite rather than accumulating unnoticed. Each ignore below is anchored to a
+# specific message and is for third-party code; the charm's own code raises
+# none. The integration env opts out (see tox.ini) because the warnings there
+# come from juju/websockets and are not ours to fix.
+filterwarnings = [
+    "error",
+    # Raised by the vendored loki_k8s charm lib and by paas_charm; the fixes
+    # belong upstream in canonical/loki-k8s-operator and canonical/paas-charm.
+    "ignore:JujuVersion.from_environ\\(\\) is deprecated:DeprecationWarning",
+    # paas_charm.charm_state calls pydantic's deprecated BaseModel.dict();
+    # the fix belongs upstream in canonical/paas-charm.
+    "ignore:The `dict` method is deprecated; use `model_dump` instead:DeprecationWarning",
+]
 
 # Linting tools configuration
 [tool.ruff]

tox.ini

@@ -91,5 +91,6 @@ commands =
            -s \
            --tb native \
            --log-cli-level=INFO \
+           --override-ini=filterwarnings= \
            {posargs} \
            {[vars]tests_path}/integration