Fixed parent session IDs being sent in request headers for session-aware providers

Fixes

• CoreFixed Cloudflare AI Gateway routing for third-party providers so non-Workers models work through the gateway's REST API.@superhighfives
• CoreFixed Anthropic models through Cloudflare AI Gateway by converting dotted model IDs like claude-haiku-4.5 to the dashed slug Anthropic expects.@superhighfives
• TerminalFixed GitHub auth for immutable OIDC subject tokens.