Slice 2l of the route-domain reorganization (#4082/#4071). Moves
webhook_routes.py into routes/webhook/, leaving a backward-compat
sys.modules shim. Pure file reorganization, no behavior change.
One source-introspection test repointed (test_api_chat_security.py).
Validate only token-supplied direct base_url values for API-token chat requests, while keeping admin-configured endpoints available for local/LAN providers.
Scope configured endpoint fallback selection to the API token owner, fail closed for unknown token owners, and preserve strict session ownership checks when resuming sessions from chat-scoped API tokens.
Add focused regression coverage for direct base_url SSRF rejection, configured endpoint fallback behavior, token-owner scoping, URL validation, and null-owner session/endpoint handling.
