fix: fail headless coder-switch commands clearly

[ump45nose]

Summary

• return a non-zero status when --message triggers a coder-switching slash command
• print a clear interactive-only error instead of silently exiting successfully
• add focused coverage for the SwitchCoder path

Why

The one-shot --message branch swallowed SwitchCoder, reported a completed run, and returned success even though commands such as /ask, /model, and /architect were not applied.

Fixes #5623

Test

• python -m pytest tests/basic/test_main.py::TestMain::test_message_coder_switch_returns_error -q

[kvnloo]

Live check on PR head 2eed4e2: --message currently excepts SwitchCoder and falls through to analytics "Completed --message" with a None exit. That is a silent success after a coder switch that never happened.

This fail-loud path is the right shape: tool_error + exit 1 + a distinct analytics reason. The test (SwitchCoder(edit_format="ask") from Coder.run) pins the headless case.

Collision: #5638 patches the same three lines with a different error string that interpolates the full --message. Prefer this wording (names the command class, not the payload). Do not merge both.

I would take this one.