fix: remove boundary sentinel from RSC responses (#81857)
## Summary
This PR fixes an issue where the test mode boundary sentinel was being
incorrectly added to RSC (React Server Component) responses. The
sentinel should only be added to HTML responses during PPR (Partial
Prerendering) test mode.
## Changes
- Added check to prevent boundary sentinel insertion when the response
is an RSC content type
- Added check to prevent boundary sentinel insertion when the request is
an RSC request
- Correctly set the response type to 'rsc' instead of 'html' for RSC
requests
## Test plan
- [x] Existing tests should pass
- [x] RSC responses should not contain the test mode boundary sentinel
- [x] HTML responses in PPR test mode should still contain the boundary
sentinel as expected
