Ephemeral buffer hardening

[johnstairs]

1. Use HTTP trailers to signal an error to the client when an error occurs when reading the named pipe in the buffer sidecar
2. Return response headers as soon as possible from relay server to avoid client timeouts
3. Improve error messages for common error cases.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no suggestions.

Comments skipped due to low confidence (2)

cli/cmd/buffer-sidecar/main.go:115

• Spelling mistake in the comment: 'listner' should be 'listener'.

// give some time for a client to connect and pass in the data that will be discarded instead of just closing the listner.

cli/cmd/buffer-sidecar/main.go:182

• Spelling mistake in the comment: 'reponse' should be 'response'.

// give some time for a client to connect and observe the empty reponse instead of just closing the listener