GOOWOO-773 Fix 500 Error Switching Shipping Rate to Manual

[zamanq]

Changes proposed in this Pull Request:

Fixes a 500 error (gla_setting_sync_error) when a merchant with WPML multi-language markets switches the shipping rate method to manual in Settings.

Closes https://linear.app/a8c/issue/GOOWOO-773/fix-500-error-switching-shipping-rate-to-manual-with-wpml-multi

Screenshots:

N/A

Detailed test instructions:

1. Activate WPML with more than one active language/currency.
2. Set the shipping rate method to automatic (or flat) and add one or more secondary markets via the Add Market screen.
3. Go to Settings and switch the shipping rate method to manual, then save.
4. The setting saves successfully - no 500 error and no gla_setting_sync_error in the response.
5. Switch back to automatic/flat and confirm shipping still syncs to Merchant Center as before.
6. Repeat step 3 on a single-language store (no WPML) and confirm behavior is unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.8%. Comparing base (7328499) to head (28142f9).

Files with missing lines	Patch %	Lines
src/MerchantCenter/MarketService.php	96.2%	1 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##             feature/multi-lingual-support   #3569     +/-   ##
=================================================================
+ Coverage                             69.4%   70.8%   +1.4%     
- Complexity                            6110    6114      +4     
=================================================================
  Files                                  956     542    -414     
  Lines                                30919   23815   -7104     
  Branches                              1881       0   -1881     
=================================================================
- Hits                                 21455   16854   -4601     
+ Misses                                9193    6961   -2232     
+ Partials                               271       0    -271     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	70.8% <96.2%> (+<0.1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/MerchantCenter/MarketService.php	98.8% <96.2%> (-0.2%)	⬇️

... and 414 files with indirect coverage changes

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