client does not accept unexpected `initial-response` on event streams

[rcoh]

The Smithy specification clearly states:

clients MUST NOT fail if an unexpected initial-request or initial-response is received.

However, the current behavior only accepts an initial-response when the client model expects it to contain valid data:

smithy-rs/codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/client/FluentBuilderGenerator.kt

Line 183 in 64144f4

return null

This is both a problem for backwards compatibility, and communication with servers other than smithy-rs