Adding a provider / route to litellm-rust

Three layers, same for every route (see ocr and realtime as references):

1. Transform contract (pure) — crates/core/src/<route>/transformation.rs: a …ProviderConfig trait (URL build + request/response transforms) + types in types.rs. No network, env, or auth.
2. Provider config (pure) — crates/providers/src/<provider>/<route>/transformation.rs: implement that trait as a const <PROVIDER>_<ROUTE>_CONFIG, mirroring the Python provider tree. Add parity unit tests.
3. HTTP / transport (the host) — crates/providers/src/<route>.rs (e.g. ocr.rs, realtime.rs): the callable fn (run_ocr, realtime). It resolves the key, builds the auth header, builds URL + transforms via the config, then does the network call. This is the only layer allowed to do I/O.

Calling: the host invokes the route fn — the Python bridge calls run_ocr; the ai-gateway server calls realtime. Register new modules in lib.rs / mod.rs, then run cargo fmt && cargo clippy --workspace -- -D warnings && cargo test --workspace.