fix: work around `immutable` header guard

[mhassan1]

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Description

This PR fixes the ResponseService.updateHeaders method to not try to mutate response headers, since it is forbidden by the fetch spec.

Resolves https://github.com/Portkey-AI/gateway/issues/1389.

Motivation

Users who install incompatible (i.e. modern) versions of @hono/node-server will end up with response errors when using Portkey.

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update
• [ ] Refactoring (no functional changes)

How Has This Been Tested?

• [ ] Unit Tests
• [ ] Integration Tests
• [x] Manual Testing

Screenshots (if applicable)

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes

Related Issues

https://github.com/Portkey-AI/gateway/issues/1389