* fix(proxy): mark session/SSO/SAML cookies Secure behind a TLS-terminating reverse proxy
litellm only sees a plain-HTTP hop when TLS terminates at a reverse proxy,
so cookie Secure attributes previously derived from (or defaulted without
regard to) the literal request scheme could be dropped in production. The
token session cookie set by every login path never carried Secure/HttpOnly/
SameSite at all.
Adds IPAddressUtils.is_request_https, a single trust-aware resolver used by
every cookie-setting call site: PROXY_BASE_URL, then X-Forwarded-Proto only
from a configured trusted proxy (general_settings.use_x_forwarded_for +
mcp_trusted_proxy_ranges), then the literal scheme. An unconfigured or
untrusted caller cannot spoof the header to force Secure on.
Resolves LIT-6748
* fix(proxy): make the shared session-cookie helper public, type new test helpers
set_session_token_cookie is imported across modules (ui_sso.py ->
proxy_server.py), so the leading underscore was misleading and breached
basedpyright's reportPrivateUsage budget with zero headroom. Also adds
missing parameter/return type annotations to the new test helper functions
per repo convention.