feat(pbs): count the ws bid stream under its own metrics endpoint

[JasonVranek]

Summary

The websocket bid stream recorded its outcome and time-to-first-bid under get_header, the same series as the HTTP fallback, so a slot served by the fallback was indistinguishable from one served by the stream. Stream attempts now count under endpoint="get_header_stream"; the fallback keeps get_header.

Four new metrics cover what a stream does that HTTP cannot:

• relay_stream_connect_latency: websocket handshake duration, with sub-second buckets
• relay_stream_updates: bid updates received per stream window
• relay_stream_invalid_frames_total: frames that could not be parsed as a bid
• relay_stream_fallback_total: handshake failures that had budget left to retry over HTTP

A handshake answered 2xx instead of 101 now reports the transport error (556) instead of the 2xx, which the stream series would otherwise read as a delivered bid. The two synthetic status codes are compile-time constants.