BUG: ManyShotJailbreakAttack accepts a negative example_count and drops examples from the end #3393

Describe the bug

ManyShotJailbreakAttack uses example_count as a slice bound (examples[:example_count]), but validate_constructor_parameters only rejects 0. A negative count is accepted and silently drops examples from the end of the list instead of selecting from the start, so example_count=-1 with the bundled dataset sends 399 examples, and with five custom examples -1 keeps four and -4 keeps one. True is accepted as 1, and non-integers fail with an unrelated error (2.5 raises TypeError: slice indices must be integers..., -5 with five examples raises the misleading Many shot examples must be provided.).

Because the check sits in validate_constructor_parameters, the REST technique API (TechniqueService.create_async) also accepts {"example_count": -1} and stores a technique that later runs with the wrong number of examples. The neighbouring bounds in that API (max_attempts_on_failure, max_turns, tree_depth, chunk_size, ...) are all rejected up front.

Steps/Code to Reproduce

import asyncio
from unittest.mock import MagicMock

from pyrit.executor.attack import ManyShotJailbreakAttack
from pyrit.prompt_target import PromptTarget
from pyrit.setup import IN_MEMORY, initialize_pyrit_async

asyncio.run(initialize_pyrit_async(memory_db_type=IN_MEMORY))

target = MagicMock(spec=PromptTarget)
examples = [{"user": f"q{i}", "assistant": f"a{i}"} for i in range(5)]

for count in (-1, -4, True):
    attack = ManyShotJailbreakAttack(objective_target=target, example_count=count, many_shot_examples=examples)
    print(count, len(attack._examples))

print(len(ManyShotJailbreakAttack(objective_target=target, example_count=-1)._examples))

Expected Results

example_count values that are not positive integers raise a ValueError naming example_count, both at construction and in validate_constructor_parameters.

Actual Results

-1 4
-4 1
True 1
399

Screenshots

N/A

Versions

• OS: Windows 11
• Python version: 3.11.9
• PyRIT version: installed from main (1979257) in editable mode