[web] Clear HTMLImageElement.src when WebImageInfo and ImgElementPlatformView are disposed - #193408

…formView are disposed (flutter#193408)

### Description

When `Image.network` is configured with `WebHtmlElementStrategy.prefer`
or `WebHtmlElementStrategy.fallback`, `NetworkImage._loadAsync`
(`loadViaImgElement`) creates an off-DOM `HTMLImageElement`, awaits
`imageElement.decode()`, and wraps it in `WebImageInfo`. When rendered
by `RawWebImage`, `ImgElementPlatformView` creates a second
`HTMLImageElement` inside an `HtmlElementView`.

Previously:
- `WebImageInfo.dispose()` was a no-op, leaving `htmlImage.src` set on
the off-DOM `HTMLImageElement` even after `ImageCache` eviction and
widget disposal. In WebKit on iOS, `imageElement.decode()` pins
full-resolution decoded RGBA8 bitmaps in non-purgeable `VM_ALLOCATE
(graphics)` memory (`PURGE=N`) until `src` is cleared or a full JS
garbage collection finalizes the detached wrapper.
- `ImgElementPlatformView` was a `StatelessWidget` that neither updated
its `<img>` element's `src` when `RawWebImage` updated in-place (for
example with `gaplessPlayback: true`) nor cleared `img.src = ''` when
unmounted.

This PR:
- Reference-counts the underlying `HTMLImageElement` across
`WebImageInfo` clones (`_HtmlImageElementHandle` /
`_HtmlImageElementRef`) and sets `htmlImage.src = ''` when the last
clone is disposed (matching the engine's
`ImageElementImageSource._doClose()` behavior in `dart:ui_web`).
- Converts `ImgElementPlatformView` to a `StatefulWidget` that tracks
the created `HTMLImageElement` via `onPlatformViewCreated`, updates
`img.src` in `didUpdateWidget`, and clears `img.src = ''` in
`dispose()`.
- Adds unit and widget tests in
`test/painting/_network_image_test_web.dart` verifying reference-counted
`WebImageInfo` disposal across clones, in-place `ImgElementPlatformView`
`src` updates, and `src` cleanup on unmount.

### Empirical Benchmark (`iPhone 17 Pro` Simulator, `iOS 26.0.1` Mobile
Safari, Flutter Web `--release`)

Stepping through 5 high-resolution JPEGs (`4672x7008` to `6215x9322`,
`677.5 MB` total uncompressed RGBA8) with
`WebHtmlElementStrategy.prefer` and unmounting:

| Metric (`com.apple.WebKit.WebContent`, `--release`) | Pre-Change
(`master`) | Post-Change (This PR) | Delta |
| :--- | :--- | :--- | :--- |
| **Post-Unmount `VM_ALLOCATE (graphics)` (before `lowMemory`)** |
`221.0 MB` | `0.0 MB` | `-221.0 MB` (`-100.0%`) |
| **Post-Unmount `phys_footprint` (before `lowMemory`)** | `542.1 MB` |
`315.7 MB` | `-226.4 MB` (`-41.8%`) |
| **Step 5 Active `phys_footprint`** | `616.5 MB` | `390.1 MB` | `-226.4
MB` (`-36.7%`) |
| **Kernel Peak `phys_footprint` (`ri_lifetime_max_phys_footprint`)** |
`1,159.8 MB` | `972.4 MB` | `-187.4 MB` (`-16.2%`) |

Fixes part of flutter#152709

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[AI contribution guidelines]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[flutter/website]: https://github.com/flutter/website
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md