refactor filter pipeline: introduce ResolvedFilterChain and FilterPipeline types

- Replace 4 separate filter params with single Arc<FilterPipeline> in llm_chat
- Add ResolvedFilterChain (filter_ids + agents) and FilterPipeline (input + output)
- Rename utils.rs to streaming.rs, extract STREAM_BUFFER_SIZE constant
- Deduplicate output filter logic via Box<dyn StreamProcessor>
- Rename upstream_path param to request_path for consistency

Made-with: Cursor

crates/common/src/configuration.rs

@@ -30,6 +30,46 @@ pub struct AgentFilterChain {
     pub input_filters: Option<Vec<String>>,
 }
 
+/// A filter chain with its agent references resolved to concrete Agent objects.
+/// Bundles the ordered filter IDs with the agent lookup map so they stay in sync.
+#[derive(Debug, Clone, Default)]
+pub struct ResolvedFilterChain {
+    pub filter_ids: Vec<String>,
+    pub agents: HashMap<String, Agent>,
+}
+
+impl ResolvedFilterChain {
+    pub fn is_empty(&self) -> bool {
+        self.filter_ids.is_empty()
+    }
+
+    pub fn to_agent_filter_chain(&self, id: &str) -> AgentFilterChain {
+        AgentFilterChain {
+            id: id.to_string(),
+            default: None,
+            description: None,
+            input_filters: Some(self.filter_ids.clone()),
+        }
+    }
+}
+
+/// Holds resolved input and output filter chains for a model listener.
+#[derive(Debug, Clone, Default)]
+pub struct FilterPipeline {
+    pub input: Option<ResolvedFilterChain>,
+    pub output: Option<ResolvedFilterChain>,
+}
+
+impl FilterPipeline {
+    pub fn has_input_filters(&self) -> bool {
+        self.input.as_ref().is_some_and(|c| !c.is_empty())
+    }
+
+    pub fn has_output_filters(&self) -> bool {
+        self.output.as_ref().is_some_and(|c| !c.is_empty())
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 #[serde(rename_all = "lowercase")]
 pub enum ListenerType {