[MOB-4320] Unit tests compilation fixes [reopened]

[d4r1091]

MOB-4320

Context

After the v147 Firefox upstream upgrade, unit tests fail to compile. The main root cause is Swift 6 strict concurrency enforcement: in Swift 6, SWIFT_STRICT_CONCURRENCY = minimal (set during the Tuist migration) has no effect — strict concurrency is unconditionally enforced, unlike Swift 5 where it acted as a suppression mechanism.`

A previous partial Swift 6 migration addressed ~30/35 production source files but explicitly deferred test file migration to unblock the v147 upgrade. Additionally, the v147 bulk file replacement overwrote several individually cherry-picked upstream commits that had already fixed Swift 6 warnings in test files (e.g., RustRemoteTabsTests, TestBrowserDB, WebServer.swift closures). Cherry-picking those fixes back was not viable due to source conflicts from the upgrade.

Approach

Investigation

• Confirmed that SWIFT_STRICT_CONCURRENCY = minimal is a no-op in Swift 6 and cannot suppress the errors
• Attempted cherry-picking upstream concurrency fixes — all conflicted due to v147 source changes
• Compared concurrency fix patterns with upstream Firefox iOS to ensure alignment

Fix Patterns Applied (aligned with upstream)

The following Swift 6 concurrency patterns were applied consistently across test and mock files:

1. @MainActor on test classes — for tests interacting with main-actor-isolated APIs
2. @unchecked Sendable — on non-final test/mock classes conforming to Sendable protocols where full conformance isn't feasible
3. nonisolated(unsafe) var — for mutable variables captured across isolation boundaries in closures
4. MainActor.assumeIsolated { } — for accessing @MainActor properties within synchronous callbacks known to run on the main thread
5. Extracting @MainActor properties before autoclosure — to avoid isolation violations in XCTAssert* calls

Other fixes

• Deleted orphaned test files referencing APIs removed in v147 (Fakespot, legacy Homepage, FeatureSwitch, etc.)
• Updated mock files to match v147 protocol changes (MockProfile, MockTabManager, MockGleanWrapper, MockLogger, MockFiles, etc.)
• Fixed Tuist target dependencies so test targets resolve their imports correctly
• Replaced the .xctestplan file with fully inline test target and skip declarations in Schemes+Ecosia.swift using Tuist's skippedTests parameter — Tuist resolves target identifiers at generation time, eliminating stale UUID issues that occurred whenever the project was regenerated
• Updated CI to Xcode 26.3, macOS 26 runner, and iPhone 17 simulator
• Switched Danger gem source from SSH to HTTPS for CI compatibility

Notes

• Upstream Firefox iOS mostly applies the same concurrency patterns (confirmed for example via FXIOS-15180 and FXIOS-14162), so hopefully these fixes will not conflict much with future upstream merges
• Even if building for tests now work with no compilation error and we can run them - tests are still failing which will be tackled separately.

CI: Temporarily disabled Merge Unit Tests

The Merge Unit Tests workflow (.github/workflows/merge_tests.yml) has been temporarily disabled by clearing its trigger branches. Running them as a merge gate at this stage produces noise rather than signal and adds significant overhead (~30+ min) with no actionable feedback.

Fixing issues and re-enabling them has been split into MOB-4384.

Before merging

Checklist

• I performed some relevant testing on a real device and/or simulator for both iPhone and iPad
• I wrote Unit Tests that confirm the expected behaviour
• I updated only the english localization source files if needed
• I added the // Ecosia: helper comments where needed
• I made sure that any change to the Analytics events included in PR won't alter current analytics (e.g. new users, upgrading users)
• I included documentation updates to the coding standards or Confluence doc, when needed

[lucaschifino]

@d4r1091 I guess you can't review it now that it's been reopened on your name 😅

From my side it's all good (well, I was the one implementing anyway 🙈) - it might be cleaner to merge this if you agree (or make any small adjustments you see) and then continue the work on the next ticket anyway.