fix: clear CLI subagent warping state on Ctrl-C stop

[oz-for-oss]

Closes #12609

Summary

• Treat Ctrl-C Stop takeovers from a CLI subagent as manual conversation cancellations instead of resumable user takeovers.
• Immediately mark the associated conversation as cancelled so the stale Warping... footer and agent controls clear when the user stops the subagent.
• Add a regression test for a monitored long-running command where Ctrl-C moves control to the user with a Stop reason and cancels the conversation.

Validation

• CARGO_BUILD_JOBS=1 cargo nextest run --manifest-path /workspace/warp/Cargo.toml -p warp -E 'test(ctrl_c_stop_takeover_cancels_monitored_long_running_command_conversation)'
• rustfmt --check /workspace/warp/app/src/ai/blocklist/block/cli_controller.rs /workspace/warp/app/src/terminal/view_tests.rs

Notes

• The initial default-concurrency cargo nextest attempt was killed by SIGKILL before tests ran in the sandbox; the single-job retry passed.