OFREP: Add missing flagKey in error responses + adding tests

[thomaspoignant]

Description

OFREP: Add missing flagKey in error responses + adding tests

Changes include

• Bugfix (non-breaking change that solves an issue)

Closes issue(s)

Resolve #1759
Resolve #1638

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	82a20c0
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/661d944a6f0c610008bc5228

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 87.09677% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 88.97%. Comparing base (2a35d96) to head (82a20c0).

Files	Patch %	Lines
cmd/relayproxy/ofrep/evaluate.go	82.60%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1760      +/-   ##
==========================================
- Coverage   88.99%   88.97%   -0.03%     
==========================================
  Files          77       79       +2     
  Lines        2989     3119     +130     
==========================================
+ Hits         2660     2775     +115     
- Misses        245      257      +12     
- Partials       84       87       +3     

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