fix(media_player,config_flow): close the batch review's O2 and O3 (t17)

Both observations from the t8-t12 batch review, settled by execution rather than
by argument.

O2 — WHICH STATES MAY CARRY A NOW-PLAYING ROW. The review read
`extra_state_attributes` and argued the row publishes the entity state verbatim,
so `now_playing_state: on` is reachable, while the README documents only
playing/paused/buffering/idle; t10's author had reported that `on` and `off`
publish no row at all. The probe settles both halves:

* the t10 account is true on the NATURAL ladder — `on` means no source reported a
  power or playback fact, and every tier that can produce a reading sets
  `reported = True` in the ladder, so the two cannot meet;
* the review is right about the RULE. It was "publish unless the state is off", an
  assumption nothing enforced: forcing the ladder to `on` (and to `standby`, which
  the README does not even mention) published `now_playing_state: on`/`standby`
  beside a real title.

So the rule is now explicit — `NOW_PLAYING_STATE_VALUES = ("playing", "paused",
"buffering", "idle")` — and a reading is dropped whole when the entity state is
anything else, exactly as `off` always dropped it. Nothing legitimate is lost: a
tier only ever answers in one of those four (`idle` for "reported, nothing
playing", which is where the foreground tier's remembered-playback bridge lands).
The README says the same thing in the table row and in prose, and
tests/test_now_playing_publication.py walks EVERY `MediaPlayerState` member with a
reading present (the four publish themselves; on/off/standby publish nothing),
then reads the four values back OUT OF THE README and compares them to the
constant, so code and docs cannot drift again.

O3 — A REJECTED APP PASTE NAMES ITS BAD LINES. The bulk paste was all-or-nothing
and reported a generic `invalid_app_map`, so a 30-row paste with one typo had to
be bisected by hand. `_app_map_parse` now returns `(map | None, problems)` where
each problem carries the 1-based line NUMBER, the line as the user typed it, and
why (empty/space-bearing package, empty name, space in the icon), and the
rejection puts them in the form's `description_placeholders["invalid_lines"]`,
which the error copy interpolates — capped at 4 named lines with the remaining
count, so the message stays readable. The map is still None when anything is
wrong (never half-stored) and the detail is cleared as soon as a paste parses.
The single-row editor is untouched and keeps its own `invalid_app_row`; a test
asserts the new per-line report and `_valid_app_row` accept and refuse the same
rows, so a paste and a typed row cannot disagree.

Verify: 794 passed / 4 skipped, 795 passed / 3 skipped (with androidtvremote2),
ruff clean.

custom_components/nvidia_shield/strings.json

@@ -120,7 +120,7 @@
       "protocol_unavailable": "The androidtvremote2 library is not installed, so the remote protocol cannot pair. Install it and restart Home Assistant, or turn on 'Continue without protocol pairing' to finish setup anyway.",
       "protocol_unreachable": "The Shield did not answer at that address. Check the address above and that the Shield is awake, then select Submit to try again — or turn on 'Continue without protocol pairing' to finish setup without it.",
       "no_state_source": "No state source selected. Choose at least one: 'Add the Nvidia Remote Protocol' or 'Read state over ADB'.",
-      "invalid_app_map": "Each line must be package=name with a non-empty package (no spaces) and a non-empty name — for example ar.tvplayer.tv=TiviMate.",
+      "invalid_app_map": "The apps text was not saved — nothing was changed. {invalid_lines} Fix those lines and select Submit again, or clear the field to go back to the built-in list.",
       "invalid_digit_delay": "The digit delay must be between 0.05 and 1.0 seconds.",
       "invalid_app_row": "A row needs a package id with no spaces and a non-empty name — for example ar.tvplayer.tv=TiviMate. An icon, if given, cannot contain spaces.",
       "unknown_app_row": "That app is no longer in the list. Pick one of the rows offered, or add it again."