* refactor(server): extract pages fallback bridge into typed helper
* test(app-router): update pages fallback codegen assertion
* refactor(server): tighten buildRequestHeaders type to Headers | null
The injected buildRequestHeaders dependency wires in
buildRequestHeadersFromMiddlewareResponse, which returns Headers | null,
but the dependency type declared a non-nullable Headers return. Tighten
the type to match reality and add a test exercising the null path where
the original request is forwarded unchanged.
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Co-authored-by: James <james@eli.cx>
Nathan Nguyen