Track Chatto v0.5.0-beta.12

No public wire changes: the only proto delta between v0.5.0-beta.11 and
v0.5.0-beta.12 is a comment-only edit to the internal
chatto.core.cache_state.v1 neighborhood.proto (NEIGHBORHOOD_IMAGES ->
ASSET_CACHE object store wording), a package chattolib does not vendor.
All public API/admin/auth/discovery/realtime proto files are byte-identical,
so the regenerated proto/ tree and _pb bindings are unchanged and the client
code needs no adaptation. The release notes' breaking changes (messages read
state, event projector consolidation) are server-internal.

Verification: ruff format+check clean (27 files, 0 pending); mypy zero new
errors vs pre-change baseline; pytest 92 passed, 2 skipped; live GetServer
against https://chat.chatto.run returns server version 0.5.0-beta.12; live
bot integration: test_bot_receives_its_own_message passed, public server
metadata passed (second-bot test skipped, no 2nd key).

Bumped version 0.5.0b11 -> 0.5.0b12.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "chattolib"
-version = "0.5.0b11"
+version = "0.5.0b12"
 description = "Async Python client library for the Chatto webchat Connect API"
 requires-python = ">=3.11"
 license = "MPL-2.0 AND Apache-2.0"