[GOOWOO-474] Performance: Avoid uncached Ads API request in NotificationManager

[jamesmorrison]

Changes proposed in this Pull Request:

Closes https://linear.app/a8c/issue/GOOWOO-474/performance-avoid-uncached-ads-api-request-in-notificationmanager

Screenshots:

Before clearing transient cache:

After clearing transient cache (i.e. the data is looked up and stored):

Detailed test instructions:

1. In terminal run: wp transient get gla_ads_highest_spend_campaign - the transient should not exist at this point so expect the output to be: Warning: Transient with key "gla_ads_highest_spend_campaign" is not set.
2. Open WP Admin, navigate to Marketing -> Google for WooCommerce and select any campaign (or create one if need be). This process should set the transient.
3. In terminal run: wp transient get gla_ads_highest_spend_campaign You should see array data set now, for example:

array (
  'campaign' => 
  array (
    'id' => 23349407739,
    'name' => 'Campaign 2025-12-11 15:09:02',
    'status' => 'enabled',
    'type' => 'performance_max',
    'targeted_locations' => 
    array (
      0 => 'GB',
    ),
    'eu_political_advertising_confirmation' => false,
    'amount' => 14.0,
    'country' => 'GB',
  ),
)

Additional details:

Add - Added transient caching for highest spend campaign.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.6%. Comparing base (dd249e1) to head (b3a140a).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #3263     +/-   ##
===========================================
- Coverage       66.8%   66.6%   -0.2%     
- Complexity      5095    5097      +2     
===========================================
  Files            867     507    -360     
  Lines          26180   20326   -5854     
  Branches        1466       0   -1466     
===========================================
- Hits           17479   13528   -3951     
+ Misses          8528    6798   -1730     
+ Partials         173       0    -173     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	66.6% <100.0%> (+0.1%)	⬆️

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

Files with missing lines	Coverage Δ
src/API/Google/AdsCampaign.php	96.5% <100.0%> (+3.9%)	⬆️

... and 362 files with indirect coverage changes

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[jjgrainger]

Thanks @jamesmorrison looks good, left some minor feedback