fix(file-provider): Prevent concurrent write crash in XPC utils

[claucambra]

Summary

On launch, it is possible for concurrent accesses to be made to some of the internal members of FileProviderXPCUtils, triggering a crash:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[1]'
*** First throw call stack:
(
	0   CoreFoundation                      0x0000000181f8abf0 __exceptionPreprocess + 176
	1   libobjc.A.dylib                     0x0000000181a1691c objc_exception_throw + 88
	2   CoreFoundation                      0x0000000181ea2ec8 -[__NSPlaceholderArray initWithObjects:count:] + 640
	3   CoreFoundation                      0x0000000181ec6310 -[NSArray initWithArray:range:copyItems:] + 364
	4   Nextcloud                           0x0000000102c664e4 _ZN3OCC3Mac20FileProviderXPCUtils23getFileProviderServicesEP7NSArrayIP21NSFileProviderManagerE + 332
	5   Nextcloud                           0x0000000102c60e10 _ZN3OCC3Mac15FileProviderXPC28connectToFileProviderDomainsEv + 236
	6   Nextcloud                           0x0000000102c45a6c _ZN3OCC3Mac12FileProvider12configureXPCEv + 344
	7   Nextcloud                           0x0000000102c5b954 _ZNK3OCC3Mac30FileProviderSettingsController16operationMessageEv + 164
	8   Nextcloud                           0x0000000102c55f94 _ZN3OCC3Mac30FileProviderSettingsControllerC2EP7QObject + 92
	9   Nextcloud                           0x0000000102c55ef0 _ZN3OCC3Mac30FileProviderSettingsController8instanceEv + 80
	10  Nextcloud                           0x0000000102a0af24 _ZN3OCC11ApplicationC2ERiPPc + 7088
	11  Nextcloud                           0x0000000102979d44 main + 160
	12  dyld                                0x0000000181aa3da4 start + 6992
)
libc++abi: terminating due to uncaught exception of type NSException

This PR synchronises writes to these data structures

Checklist

• Sign-off message is added to all commits.
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• [] Test(s) added to branch, with before/after results included in PR description, for performance improvements where applicable.
• Documentation has been updated or is not required.
• Backports requested where applicable (critical bugfixes).
• Labels added where applicable (bug/enhancement).
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AI (if applicable)

• The content of this PR was partly or fully generated using AI
  • I have read the guidelines for AI-assisted contributions.
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[claucambra]

/backport to stable-34.0 please

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-10665.zip

Digest: sha256:347d9b57bbf9d3e0a8e56cd4b0183dcd67a4ffe13045e0d2190a5a537dc3d688

To test this change/fix you can download the above artifact file, unzip it, and run it.

Please make sure to quit your existing Nextcloud app and backup your data.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Rello]

Suggested change

	private slots:
	private Q_SLOTS:

due to test error