mgr/TTLCache: fix PyObject* lifetime management and cleanup logic

mgr/TTLCache: fix PyObject* lifetime management and cleanup logic

Fix incorrect reference counting and memory retention behavior in TTLCache
when storing PyObject* values.
Previously, TTLCache::insert did not increment the reference count,
and `erase` / `clear` did not correctly decref the values, leading
to use-after-free or leaks depending on usage.

Changes:
- Move Py_INCREF from cacheable_get_python() to TTLCache::insert()
- Add `TTLCache::clear()` method for proper memory cleanup
- Ensure TTLCache::get() returns a new reference
- Fix misuse of std::move on c_str() in PyJSONFormatter

These changes prevent both memory leaks and use-after-free errors when
mgr modules use cached Python objects logic.

Fixes: https://tracker.ceph.com/issues/68989
Signed-off-by: Nitzan Mordechai <nmordech@redhat.com>
(cherry picked from commit 7fadf6a8d464456668550e1f85c6c5a86f94bb49)