Fix crash: use postValue in resetFrontDoorBridgeUrl

[wmathurin]

Summary

resetFrontDoorBridgeUrl() calls LiveData.setValue() but can be invoked from a background coroutine dispatcher (via onAuthFlowError → doCodeExchange error path). This throws IllegalStateException: Cannot invoke setValue on a background thread.

Fix: use postValue(null) instead of value = null.

Context

The bug was introduced in "Modern Login Architecture (#2653)" on 2025-01-24. It only manifests when the OAuth code exchange fails (e.g., invalid consumer key, network error during exchange) — the happy path never hits this code.

Test Plan

• Reproduced crash with invalid consumer key on React Native template app
• Verified fix resolves crash — auth error is handled gracefully

[JohnsonEricAtSalesforce]

👍🏻

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.58%. Comparing base (0ec99fc) to head (7ef093b).
⚠️ Report is 3 commits behind head on dev.

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #2908      +/-   ##
============================================
+ Coverage     66.41%   66.58%   +0.17%     
- Complexity     3099     3108       +9     
============================================
  Files           226      226              
  Lines         17774    17871      +97     
  Branches       2327     2339      +12     
============================================
+ Hits          11804    11899      +95     
+ Misses         4896     4887       -9     
- Partials       1074     1085      +11     

Components	Coverage Δ
Analytics	48.71% <ø> (ø)
SalesforceSDK	62.88% <89.81%> (+0.35%)	⬆️
Hybrid	59.30% <ø> (ø)
SmartStore	78.22% <ø> (ø)
MobileSync	82.12% <ø> (ø)

Files with missing lines	Coverage Δ
...src/com/salesforce/androidsdk/ui/LoginViewModel.kt	89.60% <100.00%> (ø)

... and 4 files with indirect coverage changes

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