ConnectError metadata missing when using connect-fastify + grpc transport

[bobaaaaa]

Describe the bug

When throwing a ConnectError with special metadata fields on the (connect-fastify) server, the metadata fields are not part of the headers anymore in the client with grpc transport.

I tried to fix the bug by myself but give up at the end. My last guess was, that the metadata fields are dropped/overridden or not correctly converted to the http header. The fastify plugin code did not receive the error http headers.

To Reproduce

I created a reproducible project: https://github.com/bobaaaaa/connect-fastify-bug

Environment:

- "fastify": "5.2.1",
- "@bufbuild/protobuf": "2.2.3",
- "@connectrpc/connect": "2.0.1",
- "@connectrpc/connect-fastify": "2.0.1"
- node 22

Additional context

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