fix(config_flow): F-A/F-A2/F-D — the discovered host and the 5555 port ride the schema KEY defaults (t33)

F-A (HIGH): _user_schema_dict re-declared vol.Optional(CONF_ADB_HOST) /
CONF_PROTOCOL_HOST inside `if "host" in defaults:` — a no-op, because
voluptuous keys compare by field name and the first (defaultless) key object
survives. The discovered address therefore never reached the form, and the
user submitted a blank host. The host keys are now declared WITH default=host
(_host_field_key), so a discovery-driven user step arrives pre-filled.

F-A2: a discovery-driven user step says the address is known
("{discovery_note}Discovered Shield at <host> — its address is already filled
in below. ") through description_placeholders, and the no_state_source
re-render keeps the prefill instead of rebuilding a blank schema.

F-D: the ADB port is declared with default=DEFAULT_ADB_PORT (5555), and a YAML
seed's port hint is carried into the form instead of being flattened to 5555.
The seed host now feeds _seed_host (same prefill, no discovery claim).

Reproduce-through-test: tests/voluptuous_mirror.py mirrors the real
voluptuous.Marker key contract (hash/eq by field name, default=UNDEFINED when
omitted), because voluptuous is not installed in either CI variant and the old
bare-key stubs could not tell a schema WITH a default from one without. The
pre-t33 discovery assertion only checked the stored attribute and stayed green
while the form rendered blank; it now reads the schema keys and fails on the
pre-fix code (8 tests reproduced red locally).

Verified: uvx --with pytest-asyncio pytest tests -q → 242 passed, 1 skipped;
uvx ruff check custom_components tests → clean.

custom_components/nvidia_shield/strings.json

@@ -3,7 +3,7 @@
     "step": {
       "user": {
         "title": "NVIDIA Shield",
-        "description": "Set up your NVIDIA Shield. Link the Android TV remote, Jellyfin, receiver, and power monitoring.",
+        "description": "{discovery_note}Set up your NVIDIA Shield. Link the Android TV remote, Jellyfin, receiver, and power monitoring.",
         "data": {
           "name": "Device Name",
           "androidtv_entity": "The media_player entity from the Android TV Remote integration. Optional for standalone setups that pair with the Nvidia Remote Protocol instead.",