fix: deliver leftover /steer as out-of-band user message (#60543)

[rainbowgore]

What does this PR do?

Fixes a /steer message being lost / delivered without its trust marker when it arrives after the final assistant turn.

/steer has three delivery paths. Two wrap the user's message in the [OUT-OF-BAND USER MESSAGE] marker so the model treats it as a genuine user instruction rather than tool output / prompt injection (apply_pending_steer_to_tool_results, and the pre-API drain in conversation_loop.py). The third — the "leftover" path, taken when a steer lands after the final assistant turn with no remaining tool batch to drain into — did not. turn_finalizer.py hands the raw steer back as result["pending_steer"], and both consumers delivered it verbatim as the next user turn, so the model saw bare /steer text instead of the marker.

On the gateway path there was a sharper failure: its downstream slash-command discard net drops any pending text starting with /, so a leftover steer whose text begins with / (e.g. steering with /stop the search) was silently discarded. Wrapping it in the marker means it no longer starts with /, so it survives and is delivered.

Approach: add one shared helper next to the existing steer helpers and route both consumers through it, rather than duplicating the wrap at each call site.

Related Issue

Fixes #60543

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• agent/agent_runtime_helpers.py — add format_leftover_steer_for_delivery() (format_steer_marker(text).strip(); the marker is the whole standalone message here, so the append-oriented leading newlines are stripped) and export it.
• cli.py — CLI REPL leftover handler now delivers format_leftover_steer_for_delivery(_leftover_steer) instead of the raw text.
• gateway/run.py — gateway leftover handler (TUI / Telegram / Slack / WhatsApp) routed through the same helper.
• tests/run_agent/test_steer_leftover_marker.py — new regression suite (6 cases).

How to Test

1. In the TUI, start a conversation that triggers parallel tool calls.
2. Send /steer "..." so it lands after the final tool batch (the leftover window).
3. Confirm it reaches the model wrapped as [OUT-OF-BAND USER MESSAGE], not as raw /steer text — and that a steer beginning with / is delivered rather than silently dropped.

Automated: tests/run_agent/test_steer_leftover_marker.py covers both delivery paths, the marker contract, and the gateway slash-command net. Each assertion was confirmed to fail against the pre-fix raw passthrough (red → green).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass — CI Python tests green on Linux (amd64 + arm64); targeted steer/finalizer/gateway suites (52) pass locally on macOS.
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.5 (arm64), Python 3.12

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — new helper is documented via docstring; no user-facing docs affected — or N/A
• N/A — no config keys added/changed (cli-config.yaml.example)
• N/A — no architecture/workflow changes (CONTRIBUTING.md / AGENTS.md)
• I've considered cross-platform impact (Windows, macOS) — change is platform-agnostic (no file I/O, process, or shell surface touched)
• N/A — no tool behavior/schema changes

Screenshots / Logs

tests/run_agent/test_steer_leftover_marker.py — 6 passed. Full pytest tests/ -q result noted in the checklist above.