fix(console): preserve inference sessions

[adamdotdevin]

Summary

• preserve the validated OpenCode session header when the legacy Zen route forwards to the managed inference gateway
• continue stripping the session and other OpenCode metadata from direct provider requests
• cover managed inference, direct provider, and empty-session behavior

Root cause

The Zen handler captured the session for legacy metrics, then unconditionally removed the header before forwarding. Requests routed through managed inference therefore emitted canonical generation events without a session ID, leaving the R2 generation table mostly null after the live-source handoff.

Impact

New managed-inference generation events retain their OpenCode session ID, allowing R2-backed stats to calculate session counts and per-session metrics correctly after deployment.

Validation

• bun test test/providerHeaders.test.ts
• bun typecheck from packages/console/app
• repository pre-push typecheck: 30 packages passed