Updates Http2VisitorInterface::OnHeaderForStream() to return a boolean. This is necessary in order to communicate when header names or values are invalid.

This change adds a test that exercises nghttp2 header validation. (Specifically, the case of an invalid trailing header name.)

This change also adds the following behavior for the OG stack, for client endpoints:
* Closes a stream that receives END_STREAM and is half-closed local.

PiperOrigin-RevId: 380816879