fix(cache-manager): await an awaitable returned by the get_or_set factory

[allen0099]

Closes #101.

Problem

get_or_set() awaited the factory only when inspect.iscoroutinefunction(factory) was true. The example in docs/APP_CACHE.md, get_or_set("user:42", lambda: load_user(42), ttl=300), passes a lambda that returns a coroutine. That coroutine went to set(), which failed with TypeError: Object of type coroutine is not JSON serializable and emitted a "never awaited" warning.

Changes

• get_or_set() calls the factory and awaits the result when inspect.isawaitable(result). This covers async functions, lambdas and functools.partial wrapping them, and async callables.
• The factory docstring says so.
• CHANGELOG [Unreleased] Fixed entry.

Tests

test_get_or_set_awaits_a_sync_callable_returning_an_awaitable is parametrized over a lambda and a functools.partial:

• The lambda case fails on the old code and passes with the fix.
• The partial case already passed, because iscoroutinefunction unwraps partials. It stays as a guard.

Local results:

• ruff check/format: clean.
• mypy (package strict, tests, scripts): clean.
• zensical build --strict: passes.
• Full suite with CACHEX_REQUIRE_LIVE_SERVERS=1 against Redis and Memcached: 720 passed, 100% coverage.