W-24237909: Restore iOS 26 UI nightly coverage

[wmathurin]

Summary

• assign the iOS 26 UI-nightly matrix entry to macos-latest
• allow the reusable workflow to schedule its Xcode 26 job instead of receiving an empty runner label
• align UI-nightly runner selection with the existing iOS unit-test nightly

Validation

• actionlint .github/workflows/ui-test-nightly.yaml
• git diff --check
• verified the iOS 26 matrix entry resolves to Xcode 26 on macos-latest

Tracking

• W-24237909

[github-actions]

	Tests	Passed	Skipped	Failed ❌️
AuthFlowTester UI Test Results all	1 ran			1 ❌

Test	Result
AuthFlowTester UI Test Results all
AuthFlowTesterUITests.xctest
LegacyLoginTests.testCAOpaque_DefaultScopes_WebServerFlow()	❌ failure

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.80%. Comparing base (90028da) to head (d672df0).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #4177      +/-   ##
==========================================
- Coverage   71.73%   67.80%   -3.94%     
==========================================
  Files         254      254              
  Lines       23027    23027              
==========================================
- Hits        16518    15613     -905     
- Misses       6509     7414     +905     

Components	Coverage Δ
Analytics	70.78% <ø> (ø)
Common	71.20% <ø> (ø)
Core	61.67% <ø> (-5.89%)	⬇️
SmartStore	73.45% <ø> (ø)
MobileSync	88.84% <ø> (ø)
see 31 files with indirect coverage changes

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[github-actions]

	Tests	Passed ☑️	Skipped	Failed ❌️
SalesforceSDKCore iOS ^18 Test Results	1045 ran	1044 ✅		1 ❌

Test	Result
SalesforceSDKCore iOS ^18 Test Results
BiometricAdvancedAuthGateTests.test_waitForSceneActive_returnsImmediatelyWhenAlreadyActive()	❌ failure