FIX: give each QRCodeConverter conversion its own image file

[MinYi Xie (ppcvote)]

Description

QRCodeConverter builds its DataTypeSerializer in __init__ (qr_code_converter.py:62). DataTypeSerializer.get_data_filename_async caches the file path after the first call (serializers.py:338-339), so every convert_async on that instance reuses the same path and segno overwrites the same PNG.

Sharing one converter instance is the normal path: AttackConverterConfig holds a single converter list, and PyRIT's own scenario tests build attacks this way (test_multilingual.py:65, test_attack_technique_factory.py:203). Reproduced with PromptSendingAttack over 3 objectives: at max_concurrency=1 all three stored conversations end up pointing at the last objective's QR code; at max_concurrency=3 the target itself receives the last objective's QR code for two of the three prompts. Nothing raises; incomplete_objectives is empty.

Fix: build the serializer inside convert_async, matching every other file-producing converter (add_text_image_converter.py:143, transparency_attack_converter.py:275). Same timestamp-based filename scheme as those converters, not a stronger uniqueness guarantee. No public API change; only the private _img_serializer attribute is removed.

Tests and Documentation

• New test: two calls on one converter must return different paths and each file must match segno.make_qr(prompt). Fails without the fix, passes with it.
• Module tests (6) and all converter/setup/scenario tests that construct a QRCodeConverter pass (199 passed, 2 skipped).
• ruff format --check, ruff check and ty check pyrit pass.
• JupyText not run: no notebook changed; the single-call usage the docs demonstrate is unaffected.
• No open PR or issue covers this: checked all 41 open PRs' changed files and searched issues for QRCodeConverter, qr_code_converter, _img_serializer and get_data_filename_async.