[web] Android WebView textZoom makes lineHeightScaleFactorOverride report the ~624.9375 sentinel, rendering all Text invisible

### Steps to reproduce

Regression variant of #178856, which was fixed by #178862 (2 Dec 2025). That fix is present in the versions below and this case still reproduces, so the sentinel-comparison problem was not fully closed.

The trigger here is an **Android WebView with `textZoom != 100`**, which is what Android's system font-size setting produces for any WebView-embedded Flutter web app. Unlike the runtime font-size change in #178856, this is wrong from first paint.

1. `flutter create mqprobe --platforms=web`, replace `lib/main.dart` with the sample below.
2. `flutter build web --release` and serve `build/web` (e.g. `python3 -m http.server 8085`).
3. On an Android emulator (Pixel_5, API 34): `adb shell settings put system font_scale 1.15`, then `adb reverse tcp:8085 tcp:8085`.
4. Load `http://localhost:8085/` in a plain Android `WebView` (`javaScriptEnabled = true`, `domStorageEnabled = true`). No `setTextZoom` call is needed — the WebView picks up the system font scale on its own.
5. Read the reported values, and note whether the red `Text` is visible.

### Expected results

`lineHeightScaleFactorOverride` should be `null`, exactly as it is at `textZoom == 100`. The user has changed their font size, not applied a text-spacing preference. `MediaQueryData.textScaler` already reports the font-size change correctly (1.15).

Per the dartdoc the value is "the system-reported height of the text, **as a multiple of the font size**", so ~625 is outside any meaningful range — WCAG 1.4.12 asks for 1.5.

### Actual results

`lineHeightScaleFactorOverride` is reported as **~624.937** at every zoom except exactly 100, and `Text` becomes unrenderable: `Text.build` multiplies its line height by that value, so an 18px line lays out ~12,936px tall and its glyphs paint thousands of pixels below the viewport. The app looks blank while icons, borders and images still draw. `RichText` is unaffected, because it never consults the override.

All measured on **Flutter 3.44.8 stable**, stock `flutter create` app, same device and build, varying only the system font scale:

| `font_scale` | `textZoom` | `lineHeightScaleFactorOverride` | `textScaler` | red `Text` visible |
|---|---|---|---|---|
| 0.85 | 85 | 624.9374824709756 | 0.85 | no |
| 1.0 | 100 | **null** | 1.0 | **yes** |
| 1.15 | 115 | 624.9347955648752 | 1.15 | no |
| 1.3 | 130 | 624.9375229225718 | 1.3 | no |

The value is a near-constant regardless of zoom direction or magnitude, which points straight at the sentinel rather than at any real measurement.

### Code sample

Code sample

```dart
import 'package:flutter/material.dart';

void main() => runApp(const ProbeApp());

class ProbeApp extends StatelessWidget {
const ProbeApp({super.key});

@override
Widget build(BuildContext context) =>
const MaterialApp(home: Scaffold(body: SafeArea(child: Probe())));
}

class Probe extends StatelessWidget {
const Probe({super.key});

@override
Widget build(BuildContext context) {
final mq = MediaQuery.of(context);
// RichText ignores the spacing overrides, so it stays readable even when
// the bug is active and every plain Text has been pushed off-screen.
final report = 'line=${mq.lineHeightScaleFactorOverride}\n'
'letter=${mq.letterSpacingOverride}\n'
'word=${mq.wordSpacingOverride}\n'
'para=${mq.paragraphSpacingOverride}\n'
'scale10=${mq.textScaler.scale(10)}\n'
'size=${mq.size.width.toStringAsFixed(0)}x${mq.size.height.toStringAsFixed(0)}';

return Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
RichText(
text: TextSpan(
text: report,
style: const TextStyle(fontSize: 15, color: Colors.black),
),
),
const SizedBox(height: 12),
const Text('PLAIN TEXT VISIBLE',
style: TextStyle(fontSize: 18, color: Colors.red)),
],
);
}
}
```

### Screenshots or Video

Screenshots / Video demonstration

Attached, one per row of the table above. Only `textZoom 100` renders the red `Text`.

### Logs

Logs

```console
[Paste your logs here]
```

### Flutter Doctor output

Doctor output

```
[!] Flutter (Channel stable, 3.44.8, on macOS 26.5.2 25F84 darwin-arm64, locale en-GB)
• Flutter version 3.44.8 on channel stable
• Framework revision 058e0af2c2, 2026-07-23 10:56:21 -0700
• Engine revision 0cd610717b
• Dart version 3.12.2
• DevTools version 2.57.0

[✓] Android toolchain - develop for Android devices (Android SDK version 36.0.0)
• Platform android-37.0, build-tools 36.0.0
• Java version OpenJDK Runtime Environment (build 21.0.10+-117844308-b1163.108)

[✓] Xcode - develop for iOS and macOS (Xcode 26.6)
[✓] Chrome - develop for the web
[✓] Network resources
```

Also reproduced on 3.41.7 and 3.44.2, both of which contain #178862.

Start with the lib/main.dart reproduction and run the listed flutter build web and Android WebView steps at multiple font scales. Compare the current behavior with #178862; done means lineHeightScaleFactorOverride remains null for non-100 textZoom and plain Text remains visible, with regression coverage added where the relevant behavior is implemented.

Written by the indexing model from the issue text.