feat(ios): set up File Provider extension (InternxtFileProvider)

[terrerox]

Summary

Adds the InternxtFileProvider NSFileProviderReplicatedExtension target to the iOS app.

• New InternxtFileProvider target implementing NSFileProviderReplicatedExtension.
• Wires InternxtSwiftCore via SPM (swift-core branch main); Package.resolved pinned.
• Apple-template stubs (FileProviderExtension, FileProviderEnumerator, FileProviderItem) with enumeration / download / upload left as TODO follow-ups.
• App Group / bundle-id placeholders pending real Internxt provisioning.
• Reuses scaffolding cherry-picked from feature/PB-5920-set-up-file-provider-ios.

This branch is stacked on fix/PB-6477-upload-download-coroutines (not master), whose Xcode/Pods state is compatible with the scaffold, so the CocoaPods-generated project.pbxproj / Podfile.lock cherry-pick applies cleanly.

[sonarqubecloud]

Quality Gate passed

Issues
31 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

[terrerox]

Closing this PR — it has been split into two stacked PRs to respect the 500-lines-per-PR limit (this combined PR was ~773 changed lines).

• [PB-5920]: File Provider extension sources (1/2) #484 — feat(ios): File Provider extension sources (1/2) — base fix/PB-6477-upload-download-coroutines (209 lines). Merge this first.
• [PB-5920]: wire File Provider target into Xcode project (2/2) #485 — feat(ios): wire File Provider target into Xcode project (2/2) — stacked on [PB-5920]: File Provider extension sources (1/2) #484 (564 lines, mostly Xcode/CocoaPods-generated).

The two PRs together reproduce exactly the same file tree as this one (no content lost). Superseding branch feature/pb-5920-ios-file-provider-extension will be deleted.