chore: refactor (chat)completions endpoints to use shared params struct (#3761)

# What does this PR do?

Converts openai(_chat)_completions params to pydantic BaseModel to
reduce code duplication across all providers.

## Test Plan
CI









---
[//]: # (BEGIN SAPLING FOOTER)
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[ReviewStack](https://reviewstack.dev/llamastack/llama-stack/pull/3761).
* #3777
* __->__ #3761

llama_stack/core/library_client.py

@@ -54,6 +54,7 @@ from llama_stack.providers.utils.telemetry.tracing import (
     setup_logger,
     start_trace,
 )
+from llama_stack.strong_typing.inspection import is_unwrapped_body_param
 
 logger = get_logger(name=__name__, category="core")
 
@@ -383,7 +384,7 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
 
         body, field_names = self._handle_file_uploads(options, body)
 
-        body = self._convert_body(path, options.method, body, exclude_params=set(field_names))
+        body = self._convert_body(matched_func, body, exclude_params=set(field_names))
 
         trace_path = webmethod.descriptive_name or route_path
         await start_trace(trace_path, {"__location__": "library_client"})
@@ -446,7 +447,8 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
         func, path_params, route_path, webmethod = find_matching_route(options.method, path, self.route_impls)
         body |= path_params
 
-        body = self._convert_body(path, options.method, body)
+        # Prepare body for the function call (handles both Pydantic and traditional params)
+        body = self._convert_body(func, body)
 
         trace_path = webmethod.descriptive_name or route_path
         await start_trace(trace_path, {"__location__": "library_client"})
@@ -493,17 +495,20 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
         )
         return await response.parse()
 
-    def _convert_body(
-        self, path: str, method: str, body: dict | None = None, exclude_params: set[str] | None = None
-    ) -> dict:
+    def _convert_body(self, func: Any, body: dict | None = None, exclude_params: set[str] | None = None) -> dict:
         if not body:
             return {}
 
-        assert self.route_impls is not None  # Should be guaranteed by request() method, assertion for mypy
         exclude_params = exclude_params or set()
-
-        func, _, _, _ = find_matching_route(method, path, self.route_impls)
         sig = inspect.signature(func)
+        params_list = [p for p in sig.parameters.values() if p.name != "self"]
+        # Flatten if there's a single unwrapped body parameter (BaseModel or Annotated[BaseModel, Body(embed=False)])
+        if len(params_list) == 1:
+            param = params_list[0]
+            param_type = param.annotation
+            if is_unwrapped_body_param(param_type):
+                base_type = get_args(param_type)[0]
+                return {param.name: base_type(**body)}
 
         # Strip NOT_GIVENs to use the defaults in signature
         body = {k: v for k, v in body.items() if v is not NOT_GIVEN}