RpcClientError loses socket error details due to hardcoded message and Cause.squash

[schickling]

Problem

When a socket error occurs in RpcClient.makeProtocolSocket, the RpcClientError is created with:

1. A hardcoded generic message "Error in socket"
2. Cause.squash(cause) which flattens the full cause tree

This results in error messages like:

Error: RpcClientError: Error in socket at ...

The underlying socket error details (like SocketCloseError with code, closeReason, or SocketGenericError with reason: "Open" | "Read" | "Write" | "OpenTimeout") are buried in the cause field and not surfaced in the error message.

Location

https://github.com/Effect-TS/effect/blob/main/packages/rpc/src/RpcClient.ts#L1010-L1014

currentError = new RpcClientError({
  reason: "Protocol",
  message: "Error in socket",  // <-- hardcoded, loses context
  cause: Cause.squash(cause)   // <-- flattens the cause tree
})

Proposed Solution

Extract socket error details and include them in the message:

const socketError = Cause.failureOption(cause)
const errorMessage = Option.isSome(socketError)
  ? `Socket ${socketError.value.reason}${
      'code' in socketError.value ? `: code ${socketError.value.code}` : ''
    }${
      'closeReason' in socketError.value && socketError.value.closeReason 
        ? ` (${socketError.value.closeReason})` 
        : ''
    }`
  : "Error in socket"

currentError = new RpcClientError({
  reason: "Protocol",
  message: errorMessage,
  cause: Cause.squash(cause)
})

This would produce messages like:

• Socket Close: code 1006
• Socket Open: connection refused
• Socket OpenTimeout

Additional Consideration

The RpcClientError class itself uses the message field as a schema property, which shadows Error.message. Consider using note (or similar) for the schema field and computing get message() from the structured fields - similar to the pattern used for other tagged errors to preserve both structured data and a human-readable message.