Completes #3732 by removing runtime URL transformations and requiring
users to provide full URLs in configuration. All providers now use
'base_url' consistently and respect the exact URL provided without
appending paths like /v1 or /openai/v1 at runtime.
Add unit test to enforce URL standardization across remote inference providers (verifies all use 'base_url' field with HttpUrl | None type)
BREAKING CHANGE: Users must update configs to include full URL paths
(e.g., http://localhost:11434/v1 instead of http://localhost:11434).
Signed-off-by: Charlie Doern <cdoern@redhat.com>