fix: preserve custom Vary headers (#75536)
## What?
Changes the handling of Vary headers in Next.js to preserve custom
headers while properly appending required Next.js internal headers using
`appendHeader` instead of `setHeader`.
## Why?
To ensure custom Vary headers set by users are not overwritten when
Next.js needs to add its own internal headers for features like RSC and
routing. This maintains proper caching behavior while supporting
user-defined variations.
## How?
- Replaced `setHeader` with `appendHeader` for Vary headers in
base-server.ts
- Added tests to verify custom Vary headers are preserved in both API
routes and App Router handlers
- Ensures Next.js internal headers (RSC, Router State Tree, Router
Prefetch) are properly appended
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Fixes #48480
Fixes #55396
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
