test(cli): stop the viewer when a test fails

[sunerpy]

Summary

A failing assertion in a cli_ui test leaves its codegraph ui server running after the test binary exits. The test unwinds past Viewer::stop, and std::process::Child neither kills nor waits when it is dropped. The test's temporary project is deleted on the way out, so the server keeps serving a directory that no longer exists. Three servers from one failed run during #298's development had been running for twelve hours.

Viewer now kills and reaps its child in Drop. stop and the panic in start both go through it, so start no longer kills by hand. Passing tests behave exactly as before.

Verification

A temporary probe test started a viewer and then panicked. It was not committed.

• On main, one server outlived the failed test (/tmp/evidence-tidy/red-probe.log).
• With this change, none did (green-probe.log).

Also run:

• cargo test -p codegraph-rs --test cli_ui: 15 passed, and no codegraph ui process is left afterwards (cli-ui.log);
• cargo clippy -p codegraph-rs --tests -- -D warnings;
• cargo fmt --all --check.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #304   +/-   ##
=======================================
  Coverage   95.05%   95.05%           
=======================================
  Files         197      197           
  Lines      108387   108387           
=======================================
  Hits       103023   103023           
  Misses       5364     5364           

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