Address Opus review findings on the device-support commits above

- Fix a real bug: airconditioner.SOUND_MODE had no exists_fn, so on
  boards (issue #319's FAC) that never report a live 'mode' value,
  entity.py's default field-presence gate silently kept the select from
  ever registering in HA -- while adapter.flatten() (what the golden/tests
  read) has no such gate, so the tests passed while documenting behavior
  the opposite of what shipped. Gate on supportedModes' presence instead.
- Add airconditioner.MDS_ABSENCE_CLEAN for the CAC-class board's
  /mds/absenceclean/vs/0 -- byte-identical shape to issue #319's
  /csi/absenceclean/vs/0, confirmed rather than guessed, closing one more
  of that board's documented coverage-gap hrefs.
- Add missing translation state labels (all 6 languages) for
  edge_lighting_mode/edge_lighting_color/indicator_light_mode's raw device
  codes, so they render as real words instead of a raw '3000K' -> '3000 K'
  fallback.
- Fix an orphaned comment above SOUND_MODE that actually described the
  unrelated DISPLAY reuse, and correct two inaccurate rationale comments:
  the sound/voice ignore reason claimed a distinction from SOUND_MODE that
  this same dump contradicts, and the /csi/* ignore block's 'same
  reasoning as air_purifier.COVERAGE' precedent only actually covers 1 of
  its 5 hrefs.
- Correct the false 'no board-token match' claim in the FAC test file and
  golden-regression docstring -- 'FAC' is a real _BOARD_TOKEN_TO_KEY entry
  (for_device_by_model alone already resolves this board); add a test
  that actually exercises that path, which nothing previously did despite
  the docstring's claim.
- Drop a tautological burner-slot test that only re-asserted what the
  golden regression test already covers via the same code path.

tests/fixtures/golden/airconditioner_cac.json

@@ -1,5 +1,6 @@
 {
   "state_keys": [
+    "absence_clean",
     "absence_power_saving_active",
     "absence_power_saving_mode",
     "air_filter_pm1_status",