feat(proxy): raise ProxyNotSetError from unset manager proxies

[allen0099]

Closes #161 (part of #113)

Problem

ProxyBase.get() raised BackendNotFoundError for every proxy. CacheManagerProxy, SessionManagerProxy and StateManagerProxy have nothing to do with a backend, so a missing session manager surfaced as "backend not found".

Change

• New ProxyNotSetError(BackendNotFoundError) in fastapi_cachex.exceptions, exported from fastapi_cachex.
  • Subclassing keeps every existing except BackendNotFoundError working.
  • The error message is unchanged ("No instance set for proxy ...").
• ProxyBase raises cls._not_set_error, which defaults to ProxyNotSetError. BackendProxy overrides it with BackendNotFoundError, so its behaviour does not change. A third-party ProxyBase subclass gets the new error.
• get_app_cache and get_state_manager now catch ProxyNotSetError rather than BackendNotFoundError around the manager proxy lookup. This makes explicit that only a missing manager triggers the lazy default.
• CHANGELOG: an Added entry. The API reference picks up the new class from the module; no other docs mention these proxies' errors.

Tests

• tests/test_proxybackend.py:
  • each manager proxy raises ProxyNotSetError, which is also a BackendNotFoundError;
  • BackendProxy still raises plain BackendNotFoundError.
• tests/test_exports.py lists ProxyNotSetError.

Mutation checks:

mutation	tests that fail
base default back to BackendNotFoundError	the three new proxy cases, plus the lazy-dependency tests (which now catch the narrower error)
drop the BackendProxy override	only the BackendProxy test
ProxyNotSetError not a BackendNotFoundError subclass	the new tests and the existing "raises when unset" tests

Ruff, mypy --strict, the full suite against live Redis and Memcached (902 passed), and zensical build --strict for both languages.