Anthropic OAuth web search now uses resolveAnthropicMetadataUserId so metadata.user_id matches the main streaming path's {session_id, account_uuid?, device_id} JSON envelope. API-key paths keep forwarding the raw session id.
Exported resolveAnthropicMetadataUserId from pi-ai. Extended the regression test to cover the OAuth shape.
Refs #2295
Forwarded the active web search session id as Anthropic Messages metadata.user_id so enterprise gateways can attribute and rate-limit search calls consistently with the main streaming path.
Added a focused Anthropic web search request-shape regression test.
Fixes#2295
roboomp