fix: CI test + mypy failures on main

- mypy: rename inner http handler in `cli.py` visualize-serve path
  from `handler` to `http_handler`. The daemon dispatcher at the top
  of `main()` already binds `handler: Callable[[Namespace], None]`,
  and reusing the name for `functools.partial(SimpleHTTPRequestHandler, ...)`
  made mypy unify the two scopes and reject both the assignment and the
  HTTPServer call site.

- test: align `test_install_qoder_config` with the canonical
  `_detect_serve_command()` pattern used by sibling tests in the same
  class (lines 528, 594). The hard-coded `"code-review-graph"` fallback
  predates PR #287, which switched the no-uvx fallback to
  `sys.executable -m code_review_graph serve` for venv robustness.
  On CI runners without `uvx` the test was asserting against a stale
  contract.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>