[SaladDay]

Claude proxy takeover could write PROXY_MANAGED into ANTHROPIC_API_KEY for providers such as OpenCode Go, triggering Claude Code's custom-key confirmation or conflicting credential warning.

Align with upstream #8111: ordinary proxy routes use only ANTHROPIC_AUTH_TOKEN in the client configuration and clear stale ANTHROPIC_API_KEY values. Provider credentials, upstream authentication, managed-account field selection, and direct mode are unchanged.

Fixes #554.

Validation:

• Both ported regressions fail before the fix and pass afterward.
• Independent blind review completed without findings; the reviewer separately passed 11 focused tests.
• 114 focused unit tests and 42 integration tests passed, covering takeover, restore, switching, and upstream authentication.
• cargo fmt --check passed; cargo clippy --lib --no-default-features completed with 14 existing warnings outside this change.
• managed_session_disable_last_app_does_not_terminate_when_status_probe_fails fails on both this branch and unchanged main (4dfe82f4), at the same process-liveness assertion. No daemon behavior changes are included.