fix(history): reject falsy non-string message IDs; drop redundant keep_count guard

The or-chain used to resolve before_msg_id / message_id evaluated
falsy non-strings (False, 0) to '' before the isinstance(…, str) check
was reached, so they silently bypassed type validation.  Replace the
chain with explicit presence-aware logic that falls through on None or
empty string (preserving alias semantics) but rejects any other
non-string value with HTTP 400.

Also remove the now-tautological 'if keep_count >= 0:' guard, which
is always True after the negativity check introduced in the parent
commit.  The DB block is unconditional and indented accordingly.

Tests: add {'before_msg_id': False}, {'before_msg_id': 0},
{'message_id': 0} to the parametrised rejection suite.

36 passed, 0 failed (Python 3.14.4 / pytest 9.1.1).

tests/test_history_truncate_validation.py

@@ -47,6 +47,7 @@ def saved_messages(database):
     {"keep_count": 1.9}, {"keep_count": -1}, {"keep_count": "bad"},
     {"keep_count": []}, {"keep_count": {}},
     {"before_msg_id": True}, {"before_msg_id": ["message"]},
+    {"before_msg_id": False}, {"before_msg_id": 0}, {"message_id": 0},
     [], None, "two", 2, False,
 ])
 def test_invalid_target_preserves_history(history_client, payload):