Fix MediaRequestAuthenticator Core Data threading issue

[kean]

Fixes an issue discovered when testing private Atomic sites.

There is a threading violation in this line that's being called from the background.

try? WPAccount.lookupDefaultWordPressComAccount(in: ContextManager.shared.mainContext)

I moved it to the main thread as a quick fix, but it really needs to be redesigned together with how session dependencies are managed in the app.

To test:

• Open a private Atomic site
• Open Media
• Verify that Xcode doesn't report any Core Data threading violations

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23189-78189d1
	Version	24.8
	Bundle ID	com.jetpack.alpha
	Commit	78189d1
	App Center Build	jetpack-installable-builds #8920

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23189-78189d1
	Version	24.8
	Bundle ID	org.wordpress.alpha
	Commit	78189d1
	App Center Build	WPiOS - One-Offs #9872

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.