selenium depends on websocket-client which installs a top-level
`websocket` package that shadows our src/websocket/ module on CI.
Renaming to ws_manager eliminates the conflict entirely — no more
sys.path hacks needed in conftest or Selenium tests.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add full pytest-cov configuration with fail_under=60% threshold,
HTML/XML report targets, and proper exclude_lines. Fix websocket
history test to use public broadcast() API instead of manually
manipulating internals. Audit confirmed 491 tests at 71.2% coverage.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>