[BREAKING] MAINT: Remove expired single-turn system prompt

[romanlutz]

Description

Remove the expired SingleTurnAttackContext.system_prompt compatibility field before the v1.0.0 release. The field was non-functional and scheduled for removal in v0.17.0; callers must use prepended_conversation instead.

This also removes the stale kwargs example and verifies that system_prompt is now rejected as unsupported.

Tests and Documentation

• uv run pytest tests\unit\executor\attack\single_turn\test_prompt_sending.py -q (59 passed)
• Ruff check and format check on changed files
• ty check on changed files
• pre-commit run --files ... on changed files
• No documentation update is needed because the removed field was already documented as non-functional and its replacement remains unchanged.

[Copilot]

Pull request overview

This PR removes the expired, non-functional SingleTurnAttackContext.system_prompt compatibility field ahead of the v1.0.0 release, enforcing prepended_conversation as the supported mechanism for system prompts in single-turn attacks.

Changes:

• Removed system_prompt from SingleTurnAttackContext, including its deprecation warning behavior.
• Updated AttackStrategy.execute_async docstring to no longer reference system_prompt as a context kwarg example.
• Updated unit tests to stop expecting a deprecation warning and to assert system_prompt is rejected as an unsupported parameter.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/unit/executor/attack/single_turn/test_prompt_sending.py	Removes deprecated-parameter expectations and adds coverage that system_prompt is now rejected.
pyrit/executor/attack/single_turn/single_turn_attack_strategy.py	Drops the deprecated system_prompt field and its deprecation-warning __post_init__.
pyrit/executor/attack/core/attack_strategy.py	Updates execute_async docstring to reflect the supported context kwargs examples post-removal.

[jsong468]

TY!