Anthropic has moved the 1M-context-window feature to General Availability,
so the context-1m-2025-08-07 beta flag is no longer accepted and now causes
400 Bad Request errors when forwarded upstream.
Remove the X-CPA-CLAUDE-1M detection and the corresponding injection of the
now-invalid beta header. Also drop the unused net/textproto import that was
only needed for the header-key lookup.