fix(auth): guard migrateRefreshToken dispatch_async against stale sessions (W-23992574)

[wmathurin]

Summary

• testDowngradeFromDPoP_* tests queue a dispatch_async(dispatch_get_main_queue(), ...) in migrateRefreshToken: then immediately clean up the auth session; the pending block fires during testLogin's waitForExpectations:timeout:20, triggering NSInternalInconsistencyException: Use SFRestAPI sharedInstance for authenticated requests (seen in Actions run 32898781351)
• Add a session-alive guard (authSessions[authSession.sceneId]) in both the outer and inner dispatch_async block so stale sessions are silently skipped
• Fix testLogin's empty failure block to surface errors via XCTFail — previously silent failures masked real timeouts

Test plan

• Ran SFUserAccountManagerTests locally: 24 tests, 0 failures (including testLogin passing in ~0.9 s with a real network token refresh)
• Validated against CI failure scenario: testDowngradeFromDPoP_* tests run before testLogin with no crash

[github-actions]

	1 Warning
⚠️	Static Analysis found an issue with one or more files you modified. Please fix the issue(s).

Clang Static Analysis Issues

File	Type	Category	Description	Line	Col
SFUserAccountManager	Nullability	Memory error	Null passed to a callee that requires a non-null 2nd parameter	1748	15
SFUserAccountManager	Nullability	Memory error	Null passed to a callee that requires a non-null 2nd parameter	1763	15
SFUserAccountManager	Nullability	Memory error	nil passed to a callee that requires a non-null 1st parameter	2439	21
SFUserAccountManager	Nullability	Memory error	nil passed to a callee that requires a non-null 2nd parameter	2663	13

Generated by 🚫 Danger

[github-actions]

	Tests	Passed ✅	Skipped	Failed
SalesforceSDKCore iOS ^18 Test Results	1009 ran	1009 ✅

Test	Result
No test annotations available

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.21%. Comparing base (be15ae7) to head (8e2f179).

Files with missing lines	Patch %	Lines
...SDKCore/Classes/UserAccount/SFUserAccountManager.m	50.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #4146      +/-   ##
==========================================
- Coverage   71.62%   69.21%   -2.41%     
==========================================
  Files         254      254              
  Lines       22837    22839       +2     
==========================================
- Hits        16356    15809     -547     
- Misses       6481     7030     +549     

Components	Coverage Δ
Analytics	70.78% <ø> (ø)
Common	70.79% <ø> (-0.10%)	⬇️
Core	63.75% <50.00%> (-3.61%)	⬇️
SmartStore	73.45% <ø> (ø)
MobileSync	88.84% <ø> (ø)

Files with missing lines	Coverage Δ
...SDKCore/Classes/UserAccount/SFUserAccountManager.m	60.11% <50.00%> (-6.52%)	⬇️

... and 26 files with indirect coverage changes

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[github-actions]

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

Test	Result
No test annotations available

[sfdctaka]

LGTM!