fix(acp): clear stale interrupted prompt when a new turn starts

[hashbender]

What does this PR do?

Fixes a state-machine bug in the ACP adapter where a /steer on an idle session can resurrect a prompt the user cancelled and moved on from.

state.interrupted_prompt_text powers the "Zed-interrupt salvage" feature (replay an interrupted prompt when /steer arrives right after a client cancel, PR NousResearch#18258). But it is:

• set only in cancel() — when a turn is actively running, and
• cleared only inside the /steer salvage block.

No other path clears it. So this sequence leaks a stale value:

1. Session is running prompt P1 = "refactor the auth module". User hits ESC → cancel() stores interrupted_prompt_text = "refactor the auth module". Turn ends.
2. User sends an ordinary prompt P2 = "what's the weather". It runs to completion. interrupted_prompt_text is still "refactor the auth module" — the normal turn never cleared it.
3. User later types /steer be concise on the idle session. The salvage block sees the stale value as truthy and runs:
   "refactor the auth module\n\nUser correction/guidance after interrupt: be concise" as a real LLM turn.

The user expected to nudge the current (nonexistent) turn; instead the agent silently re-executes an abandoned task from two turns ago.

Fix: clear interrupted_prompt_text at the turn-start transition (where is_running flips to True), so starting any real turn drops the stale salvage buffer. The /steer salvage path already consumes and clears the field before this point, so legitimate immediate-salvage (steer directly after a client cancel, with no intervening turn) is unaffected.

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• acp_adapter/server.py — in prompt(), set state.interrupted_prompt_text = "" in the is_running = True turn-start transition.
• tests/acp_adapter/test_acp_commands.py — test_acp_normal_turn_clears_stale_interrupted_prompt: a running-cancel value is cleared by a completed normal turn, and a subsequent /steer runs only the steer text.

How to Test

1. scripts/run_tests.sh tests/acp_adapter/test_acp_commands.py -q — 7 tests pass, including the existing salvage test test_acp_steer_after_zed_interrupt_replays_interrupted_prompt_with_guidance.
2. Proof the fix is load-bearing: revert the acp_adapter/server.py hunk and rerun -k clears_stale → the new test fails (interrupted_prompt_text still holds the abandoned prompt, and /steer resurrects it); restore → it passes.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(acp):)
• I searched for existing PRs to make sure this isn't a duplicate (searched interrupted_prompt_text — none; open PR fix(acp): ignore idle cancel before next prompt NousResearch/hermes-agent#50461 touches idle-cancel cancel_event poisoning, a different field/path, and leaves this bug)
• My PR contains only changes related to this fix
• I've run the test suite and all tests pass
• I've added tests for my changes
• I've tested on my platform: macOS 15

Documentation & Housekeeping

• Documentation (README, docs/, docstrings) — N/A (inline comment explains the clear)
• cli-config.yaml.example — N/A
• CONTRIBUTING.md / AGENTS.md — N/A
• Cross-platform impact — N/A (scripts/check-windows-footguns.py clean)
• Tool descriptions/schemas — N/A

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Mirror-of: NousResearch#56624
NousResearch#56624