[Infra] Promote internal staging to main

Replace direct ``request.url.path`` reads in auth, ACL, routing, and
audit-log decisions with ``get_request_route(request)`` — the helper
already added in ``auth/auth_utils.py`` that returns the ASGI
``scope["path"]`` with ``root_path`` stripped. Starlette reconstructs
``url.path`` from the Host header; ``scope["path"]`` is uvicorn's
parse of the request line and matches what FastAPI dispatches on, so
it's the authoritative route for any decision that should agree with
the actual handler.

Sites:
- _experimental/mcp_server/auth/user_api_key_auth_mcp.py
- management_endpoints/mcp_management_endpoints.py
- vector_store_endpoints/utils.py
- pass_through_endpoints/pass_through_endpoints.py
- auth/route_checks.py
- litellm_pre_call_utils.py
- spend_tracking/spend_management_endpoints.py
- common_utils/http_parsing_utils.py
- management_helpers/utils.py
- health_endpoints/_health_endpoints.py

Adds regression tests in tests/proxy_unit_tests/test_proxy_routes.py
that construct a Request with scope["path"] set to a benign route and
the Host header crafted so url.path would resolve differently; each
site's decision is asserted against scope["path"].

Move the ``from litellm.proxy.auth.auth_utils import get_request_route``
imports added in the prior commit back to the function bodies that use
them. The module-level form participates in a long-standing import
cycle through ``auth_utils -> _types -> ...`` and was flagged by CodeQL
on the PR; the lazy form matches the pattern the proxy already uses
for ``user_api_key_auth`` and related helpers elsewhere in these files.

Also drop the ``RouteChecks._is_assistants_api_request`` delegation in
``_get_metadata_variable_name`` introduced in the prior commit — the
delegation pulled ``RouteChecks`` into the same cycle, and the call
site reuses the resolved route for its other branches, so inlining
the substring check is both cycle-free and avoids a redundant second
``get_request_route`` call.

Comment in test_proxy_routes.py acknowledges that the two MCP table
entries exercise ``get_request_route`` directly rather than the full
production handler (which needs ASGI scope + MCP state to invoke).

chore(proxy): route path-dependent call sites through get_request_route