ref: migrate PagerPageHolder from RxJava to Coroutines

[nonproto]

What: Removed RxJava imports (Observable, Subscription, Schedulers) from PagerPageHolder.kt and converted readImageHeaderSubscription to a Coroutine Job.
Why: Modernization and reducing the footprint of the RxJava dependency.
Nuances: Translated .subscribeOn(Schedulers.io()) to withContext(Dispatchers.IO) and .observeOn(AndroidSchedulers.mainThread()) to withContext(Dispatchers.Main). Handled stream lifecycle using awaitCancellation() inside a try/finally block to mirror .flatMap { Observable.never<Unit>() }. Handled blocking IO operations appropriately. Re-throw CancellationException and updated PR comments. Removed patch_pr.py.

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PR created automatically by Jules for task 15699169375434068070 started by @nonproto

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[gemini-code-assist]

Code Review

This pull request migrates the image header reading logic in PagerPageHolder from RxJava to Kotlin Coroutines. Key changes include replacing Subscription with Job, using scope.launch and withContext for asynchronous operations, and implementing awaitCancellation() within a try-finally block to manage stream lifecycles. A review comment suggests adding explicit imports for CancellationException and Job to improve code readability by avoiding fully qualified names.

[gemini-code-assist]

Avoid using fully qualified names for common kotlinx.coroutines APIs. Instead, add imports to improve code readability and reduce boilerplate.

Suggested change

	import kotlinx.coroutines.Job
	import kotlinx.coroutines.CancellationException
	import kotlinx.coroutines.Job

References

1. Avoid using fully qualified names for common kotlinx.coroutines APIs. Instead, add imports to improve code readability and reduce boilerplate.