Type component attributes with a covariant Attributes alias

Twin of the children fix: builders annotated ``attributes`` as
``list[HTMLAttribute] | None``, and ``list`` is invariant, so passing the
``list[tuple[str, str]]`` a caller naturally writes was a type error.

Add ``Attributes = Sequence[HTMLAttribute]`` (covariant) and use it for the
``attributes`` parameter of every builder. Locals that get appended/concatenated
stay a concrete ``list[HTMLAttribute]`` via the new ``as_attributes()``
normaliser, mirroring ``as_children()`` — builders call it once up front so
``attributes + [...]`` keeps working on a real list.

Pyright on common/components drops 45 → 42; the remaining errors are all
pre-existing and unrelated (django-stubs model access, the ``mark_safe``
``_Wrapped`` return type, and the separate ``FilterSelect`` options-list
invariance). Full suite green (443).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/components/search_select.py

@@ -22,7 +22,7 @@ from collections.abc import Callable, Iterable
 from typing import TypedDict
 
 
-from common.components.core import Element, HTMLAttribute, Media, Node
+from common.components.core import Attributes, Element, HTMLAttribute, Media, Node
 from common.components.primitives import Div, Input, Pill, Span, Template
 
 # Both comboboxes are wired by search_select.js.
@@ -176,9 +176,9 @@ def _option_row(option: SearchSelectOption) -> Node:
 
 def _combobox_shell(
     *,
-    container_attributes: list[HTMLAttribute],
+    container_attributes: Attributes,
     pills: Node,
-    search_attributes: list[HTMLAttribute],
+    search_attributes: Attributes,
     options_children: list[Node],
     always_visible: bool,
     items_visible: int,