fix(agent): restore temporary system_prompt override for invocations

[giulio-leone]

Bug

After #289 removed kwargs spreading, there is no way to temporarily override the system prompt for a single agent invocation. Setting agent.system_prompt permanently changes it, losing the original.

Reported in: #344

Root cause

The system_prompt parameter was removed from __call__, invoke_async, and stream_async as part of the kwargs deprecation. Previously it flowed through kwargs into invocation_state, but it was never properly consumed as a temporary override — it just happened to be accessible.

Fix

• Add system_prompt as a proper keyword parameter to __call__, invoke_async, and stream_async
• Use a _UNSET sentinel to distinguish "not provided" from explicit None
• Save and restore the original system prompt in a try/finally block around the invocation
• The override is active only during the invocation — the original is always restored, even on exception

Usage

agent = Agent(system_prompt="You are a helpful assistant")

# Temporary override — original prompt restored after call

# Permanent change
agent.system_prompt = "New permanent prompt"

Testing

• Updated test_agent__call__passes_invocation_state to verify system_prompt is now a proper parameter
• Added test_agent__call__temporary_system_prompt_override: verifies the override is active during the call and the original is restored after
• All 114 agent tests pass

Fixes #344

[giulio-leone]

Closing to reduce PR volume. Happy to resubmit individually if the team finds this fix useful.