De-duplicate logic between REST API and URL Metrics post type

[ShyamGadde]

Summary

Fixes #1858

Relevant technical choices

• Introduced OD_URL_Metrics_Post_Type::update_post( string $slug, string $post_title, OD_URL_Metric_Group_Collection $url_metric_group_collection ) to replace store_url_metric(), eliminating redundant logic.
• Updated REST API to use update_post() instead of store_url_metric(), ensuring the existing OD_URL_Metric_Group_Collection is passed directly.
• Moved store_url_metric() to Optimization_Detective_Test_Helpers for test compatibility.
• Refactored affected tests to align with the new method structure.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.63%. Comparing base (d21b74d) to head (010f621).

Files with missing lines	Patch %	Lines
...lugins/optimization-detective/storage/rest-api.php	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1867      +/-   ##
==========================================
- Coverage   66.64%   66.63%   -0.01%     
==========================================
  Files          88       88              
  Lines        7015     7011       -4     
==========================================
- Hits         4675     4672       -3     
+ Misses       2340     2339       -1     

Flag	Coverage Δ
multisite	66.63% <93.75%> (-0.01%)	⬇️
single	37.29% <0.00%> (+0.02%)	⬆️

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Co-authored-by: ShyamGadde <[email protected]>
Co-authored-by: westonruter <[email protected]>

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