* refactor(rust): give Messages configs typed litellm params
* feat(rust): port _normalize_system_role_messages for Messages hosts
mid_conversation_system mirrors Python's module function for function: a
leading run of system turns is hoisted into the top-level system, a later
turn stays in place when the model map flags supports_mid_conversation_system
and otherwise becomes a user turn where it was, never between a tool_use and
its tool_result, and billing blocks are stripped from the hoisted system. The
capability joins MessagesModelCapabilities and the bridge projection. Azure AI
uses it in place of fold_system_role_messages, which hoisted every turn.
* fix(rust): keep the Bedrock runtime endpoint behind a blank api_base
* refactor(rust): import LitellmParams from its crate instead of a re-export
* refactor(rust): template the Messages request decode errors
Both decode failures go through ErrorDetail::invalid with the subject and the
serde error as source instead of a preformatted sentence.
* refactor(rust_bridge): read the module globals the param specs name
The Messages host no longer keeps its own list of litellm globals; a spec
whose spellings the call leaves out names the global to read, so the next
family that has one needs no host change.
* refactor(rust): give each route lookup Python's implicit None
ProviderConfigManager's per-route methods list only the providers the route
serves and fall through to None; the Rust lookups now do the same, so a new
LlmProviders variant touches only the route that serves it.
