Use dd-sts OIDC token exchange for Datadog coverage upload

[ManuelPalenzuelaDD]

Follows up on #2773 — replaces the DD_API_KEY repo secret with dd-sts OIDC token exchange, following the team's security policy (no repo secrets).

The dd-sts policy datadog-operator was added in DataDog/dd-source#403647.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.75%. Comparing base (284ce70) to head (23d4e39).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2871      +/-   ##
==========================================
+ Coverage   39.21%   39.75%   +0.53%     
==========================================
  Files         314      315       +1     
  Lines       27301    28012     +711     
==========================================
+ Hits        10707    11137     +430     
- Misses      15803    16061     +258     
- Partials      791      814      +23     

Flag	Coverage Δ
unittests	39.75% <ø> (+0.53%)	⬆️

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see 10 files with indirect coverage changes

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[datadog-official]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 39.81%

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