Issue:
Bug: docs: Adding Unified error codes to the API - Ref 



Adding Unified error codes to the API - Ref 

Pull request description:
## Type of Change
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- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [X] Documentation
- [ ] CI/CD

## Description
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Helps closing - #6318 
### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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2. `crates/router/src/configs`
3. `loadtest/config`
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## Motivation and Context
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## How did you test it?
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## Checklist
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- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:

Patch: