* fix(history): reject implicit and coerced truncation targets
* 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).
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Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>
