context_servers: Fix tool/list and prompt/list (#20387)

There are two issues with too/list and prompt/list at the moment. We
serialize params to `null`, which is not correct according to
context_server spec. While it IS allowed by JSON RPC spec to omit
params, it turns out some servers currently missbehave and don't respect
this. So we do two things

- We omit params if it would be a null value in json.
- We explicitly set params to {} for tool/list and prompt/list to avoid
it being omitted.

Release Notes:

- N/A

crates/context_servers/src/protocol.rs

@@ -113,7 +113,10 @@ impl InitializedContextServerProtocol {
 
         let response: types::PromptsListResponse = self
             .inner
-            .request(types::RequestType::PromptsList.as_str(), ())
+            .request(
+                types::RequestType::PromptsList.as_str(),
+                serde_json::json!({}),
+            )
             .await?;
 
         Ok(response.prompts)
@@ -125,7 +128,10 @@ impl InitializedContextServerProtocol {
 
         let response: types::ResourcesListResponse = self
             .inner
-            .request(types::RequestType::ResourcesList.as_str(), ())
+            .request(
+                types::RequestType::ResourcesList.as_str(),
+                serde_json::json!({}),
+            )
             .await?;
 
         Ok(response)