Ramsundar - Prevent duplicate PR entries in PR Grading Dashboard (backend)

[Ram-blip]

Description

Related PRS (if any):

This backend PR is related to the #4921 frontend PR.

Main changes explained:

• Updated normalizePrNumbers to strip all whitespace for consistent key comparison
• Integrated this check into validateGradingEntry as a defensive layer before the data reaches mergeGradedPrs

How to test:

1. check into current branch
2. do npm install and ... to run this PR locally
3. Clear site data/cache
4. log as admin user
5. Go to /pr-grading-dashboard
6. Click "Add new" button and check with existing Pr's for the particular user

Screenshots or videos of changes:

https://www.loom.com/share/cb7835414e18423f80d845daf0fcb0ba

Note:

Include the information the reviewers need to know.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud