acp: add /model command for per-session model switching

[adityavkk]

Summary

• Add a new ACP slash command /model so users can inspect the current session model and switch to a different model without restarting the CLI process.
• Wire model switching into the agent runtime by updating set_session_model (previously a no-op) and the slash-command prompt handler to apply model changes to the active session agent.
• Add focused ACP test coverage for slash command help/text generation, command routing, and runtime model updates.

Why

Switching models currently requires restarting the CLI session, which slows down iterative workflows (e.g., moving from a fast model to a stronger one mid-session). This change enables explicit in-session switching while keeping the command flow consistent with existing slash-command patterns like /confirm.

Validation

• make lint
• make test
• make test-binary
• uv run pytest tests/acp/test_slash_commands.py tests/acp/test_mode_switching.py tests/acp/test_agent.py

[VascoSch92]

Hey @adityavkk

we are implementing that at the sdk level :-) see here: OpenHands/software-agent-sdk#2192
I will love your opinion :-)