fix: image loading when using markdown img and bigImageUrl

[jmattheis]

Fixes #333

When receiving a message with the same image in the markdown body and in the extras client::notification.bigImageUrl, then there is a clash on the file system.

One request succeeds and the other fails with the following error. This commit ensures that there is only one coil image loader instance, so that there shouldn't be file system race conditions.

WebSocket(1): received message {"id":845,"appid":21,...}
Failed - http://192.168.178.2:8000/1.jpg?v=1718369188 - java.lang.IllegalStateException: closed
java.lang.IllegalStateException: closed
    at okio.RealBufferedSource.rangeEquals(RealBufferedSource.kt:466)
    at okio.RealBufferedSource.rangeEquals(RealBufferedSource.kt:130)
    at coil.decode.SvgDecodeUtils.isSvg(DecodeUtils.kt:19)
    at coil.decode.SvgDecoder$Factory.isApplicable(SvgDecoder.kt:104)
    at coil.decode.SvgDecoder$Factory.create(SvgDecoder.kt:99)
    at coil.ComponentRegistry.newDecoder(ComponentRegistry.kt:100)
    at coil.intercept.EngineInterceptor.decode(EngineInterceptor.kt:197)
    at coil.intercept.EngineInterceptor.access$decode(EngineInterceptor.kt:42)
    at coil.intercept.EngineInterceptor$execute$executeResult$1.invokeSuspend(EngineInterceptor.kt:131)
    at coil.intercept.EngineInterceptor$execute$executeResult$1.invoke(EngineInterceptor.kt)
    at coil.intercept.EngineInterceptor$execute$executeResult$1.invoke(EngineInterceptor.kt)
    at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:78)
    at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:167)
    at kotlinx.coroutines.BuildersKt.withContext(Unknown Source)
    at coil.intercept.EngineInterceptor.execute(EngineInterceptor.kt:130)
    at coil.intercept.EngineInterceptor.access$execute(EngineInterceptor.kt:42)
    at coil.intercept.EngineInterceptor$execute$1.invokeSuspend(EngineInterceptor.kt)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
    at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:108)
    at kotlinx.coroutines.internal.LimitedDispatcher$Worker.run(LimitedDispatcher.kt:115)
    at kotlinx.coroutines.scheduling.TaskImpl.run(Tasks.kt:103)
    at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:584)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:793)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:697)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:684)
Successful (NETWORK) - http://192.168.178.2:8000/1.jpg?v=1718369188

---

The problem is reproducible: host a big image (20mb or more nearly trigger the bug 100%) on a server. When this message

{
  "title": "Title",
  "message": "![](http://192.168.178.2:8000/1.jpg?v=VERSION)",
  "priority": 5,
  "extras": {
    "client::display": {
      "contentType": "text/markdown"
    },
    "client::notification": {
      "bigImageUrl": "http://192.168.178.2:8000/1.jpg?v=VERSION"
    }
  }
}

is viewed on android, then either the bigImageUrl or the markdown image doesn't load correctly. Caching suppresses the issue, so replace VERSION with something unique, so the bug is triggered on every POST.

cat msg.json| sed -E "s/VERSION/`date +%s%N`/g" |http POST ':8080/message?token=TOKEN'

---

After this commit, I couldn't reproduce the problem anymore.

[jmattheis]

@cyb3rko Would you mind reviewing this PR, it's related to #337.

[cyb3rko]

LGTM.
But the edge cases were not tested because I do not have that kind of setup for testing it atm, I trust you there :)

I've added a smaller comment

[jmattheis]

@cyb3rko Thanks for the review, what smaller comment do you mean?