`_reconfigure_provider()` handled `image_gen_plugin_name` in both
branches (no-env-vars early return and post-env-vars) but never mirrored
the same handling for `video_gen_plugin_name`. The first-time
`_configure_provider()` path correctly routes to
`_select_plugin_video_gen_provider()`; reconfigure forgot to.
Repro:
1. Enable video_gen in `hermes tools` → Configure for All Platforms.
2. Go back into `hermes tools` → Reconfigure tool → Video Generation.
3. Pick xAI (with XAI_API_KEY already set).
4. Hit Enter at the "keep current key?" prompt.
Expected: `video_gen.provider: xai` written to config.yaml.
Actual: function returns silently; no `video_gen:` block ever written;
`video_generate` tool fails with "No video generation backend is
configured."
Fix: add the missing `video_gen_plugin_name` branch in both code paths
of `_reconfigure_provider()`, mirroring the existing
`image_gen_plugin_name` handling and the first-time configure logic.
Tests: `tests/hermes_cli/test_video_gen_picker.py` covers both branches
(env-vars-set keep-current and no-env-vars paths).
