feat(api): introduce /rerank (#2940)

# What does this PR do?
Context: https://github.com/meta-llama/llama-stack/issues/2937

The API design is inspired by existing offerings, but not exactly the
same:
* `top_n` as the parameter to control number of results, instead of
`top_k`, since `n` is conventional to control number
* `truncation` bool instead of `max_token_per_doc`, since we should just
handle the truncation automatically depending on model capability,
instead of user setting the context length manually.
* `data` field in the response, to be consistent with other OpenAI APIs
(though they don't have a rerank API). Also, it is one less name to
learn in the API.

## Test Plan

Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

llama_stack/apis/inference/inference.py

@@ -473,6 +473,28 @@ class EmbeddingsResponse(BaseModel):
     embeddings: list[list[float]]
 
 
+@json_schema_type
+class RerankData(BaseModel):
+    """A single rerank result from a reranking response.
+
+    :param index: The original index of the document in the input list
+    :param relevance_score: The relevance score from the model output. Values are inverted when applicable so that higher scores indicate greater relevance.
+    """
+
+    index: int
+    relevance_score: float
+
+
+@json_schema_type
+class RerankResponse(BaseModel):
+    """Response from a reranking request.
+
+    :param data: List of rerank result objects, sorted by relevance score (descending)
+    """
+
+    data: list[RerankData]
+
+
 @json_schema_type
 class OpenAIChatCompletionContentPartTextParam(BaseModel):
     """Text content part for OpenAI-compatible chat completion messages.
@@ -1131,6 +1153,24 @@ class InferenceProvider(Protocol):
         """
         ...
 
+    @webmethod(route="/inference/rerank", method="POST", experimental=True)
+    async def rerank(
+        self,
+        model: str,
+        query: str | OpenAIChatCompletionContentPartTextParam | OpenAIChatCompletionContentPartImageParam,
+        items: list[str | OpenAIChatCompletionContentPartTextParam | OpenAIChatCompletionContentPartImageParam],
+        max_num_results: int | None = None,
+    ) -> RerankResponse:
+        """Rerank a list of documents based on their relevance to a query.
+
+        :param model: The identifier of the reranking model to use.
+        :param query: The search query to rank items against. Can be a string, text content part, or image content part. The input must not exceed the model's max input token length.
+        :param items: List of items to rerank. Each item can be a string, text content part, or image content part. Each input must not exceed the model's max input token length.
+        :param max_num_results: (Optional) Maximum number of results to return. Default: returns all.
+        :returns: RerankResponse with indices sorted by relevance score (descending).
+        """
+        raise NotImplementedError("Reranking is not implemented")
+
     @webmethod(route="/openai/v1/completions", method="POST")
     async def openai_completion(
         self,