fix(composer): reject config keys that shadow plugin state

Five review findings, plus the two bandit reported.

Config variable keys were checked against an identifier regex only.
Python keywords slipped past it and were caught downstream by ast.parse,
but reported as

    Generated code has a syntax error: invalid syntax (<unknown>, line 17)

which names neither the field nor the value. They are now refused by
name, soft keywords ('match', 'case') included.

Worse, a key matching a BasePlugin attribute generated *valid* code that
silently clobbered plugin state. 'config' is the sharp one: the
assignment lands immediately after super().__init__(), so

    self.config = config.get("config", "x")

replaces the plugin's config dict with a string, and every later
self.config.get(...) fails at runtime. Refused now, along with logger,
display_manager, cache_manager, plugin_id, enabled, self and the
lifecycle method names. A test pins the ordering assumption that reserved
list rests on, so it fails if config vars are ever emitted before
super().__init__() instead.

Also:

- The silent `except Exception: pass` around manifest parsing now logs.
  It left "partial import produced nothing" indistinguishable from a
  malformed manifest. (bandit B110)
- list_plugins() called iterdir() on a directory that may not exist --
  a fresh install or a bad path returned 500 instead of an empty list.
- metadata.id is stripped in the two route handlers, matching
  _generate_plugin_files, which strips before validating. Without it
  " my-plugin " generated fine and then failed the id check at install,
  reading as a generator bug.
- The jinja Environment's autoescape=False now says why: these templates
  emit Python, and escaping a quote to &#34; inside generated code would
  break it. Safety comes from the values instead -- _safe_int, _rgb_expr
  and _reject_source_breaking, all covered by the injection suite.
  (bandit B701, marked nosec with that rationale)

bandit on composer.py: 2 findings -> 0.

Verified: 156 tests across the two composer suites. Removing either new
key check fails 9.

Not reproduced: the suggestion to emit `pass` so a conditional block is
never empty. 'line' and 'divider' render through a different template
branch and 'section' emits nothing at all, so no element type available
here produces an `if width >= N:` with an empty body. Left alone rather
than changing template output speculatively.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

test/test_composer_code_injection.py

@@ -115,3 +115,57 @@ def test_the_generated_module_still_has_no_top_level_statements():
     src = _generated(_payload(elements=[el]))
     assert not _module_level_code(src)
     assert "(1, 2, 3)" in src
+
+
+# --- config variable keys ---------------------------------------------------
+
+def _with_key(key):
+    return {"metadata": dict(BASE_META), "elements": [],
+            "dataModel": {"configVars": [{"key": key, "type": "string",
+                                          "default": "x", "label": "L"}]}}
+
+
+@pytest.mark.parametrize("key", ["class", "def", "import", "None", "True",
+                                 "lambda", "pass", "match", "case"])
+def test_a_keyword_config_key_is_named_in_the_error(key):
+    """ast.parse already rejected these, but as an unhelpful line number.
+
+    "Generated code has a syntax error: invalid syntax (line 17)" tells the
+    user nothing about which field to fix.
+    """
+    with pytest.raises(C.ComposerInputError) as exc:
+        _generated(_with_key(key))
+    assert key in str(exc.value) and "keyword" in str(exc.value).lower()
+
+
+@pytest.mark.parametrize("key", ["config", "logger", "display_manager",
+                                 "cache_manager", "plugin_id", "enabled",
+                                 "self", "update", "display"])
+def test_a_reserved_attribute_config_key_is_refused(key):
+    """These generate *valid* Python that silently clobbers plugin state.
+
+    The worst is `config`: the assignment lands right after super().__init__(),
+    so `self.config = config.get("config", "x")` replaces the plugin's config
+    dict with a string and every later self.config.get(...) fails at runtime.
+    """
+    with pytest.raises(C.ComposerInputError) as exc:
+        _generated(_with_key(key))
+    assert key in str(exc.value) and "reserved" in str(exc.value).lower()
+
+
+@pytest.mark.parametrize("key", ["brightness", "my_var", "_private", "x1",
+                                 "update_interval_seconds"])
+def test_ordinary_config_keys_are_still_accepted(key):
+    src = _generated(_with_key(key))
+    assert f"self.{key} = config.get(" in src
+
+
+def test_the_generated_config_assignment_does_not_precede_super_init():
+    """Guards the reasoning behind the reserved list, not just the list."""
+    src = _generated(_with_key("brightness"))
+    body = src.splitlines()
+    super_at = next(i for i, l in enumerate(body) if "super().__init__(" in l)
+    assign_at = next(i for i, l in enumerate(body) if "self.brightness = config.get(" in l)
+    assert assign_at > super_at, (
+        "config vars are assigned before super().__init__(); the reserved-name "
+        "list assumes they land after it")