sdk, agent: review fixes (T18R)

- The serve loop ends the process after EOF or exit_after_response,
  also with background tasks or listeners.
- One schema generator per direction for the whole document, so types
  with the same name get distinct names.
- The agent gives the complete error (code, message, data) to the
  model, writes SKIP only after a successful request, caps tool output
  at 64 KiB, writes each history line in one write, and skips a damaged
  line.
- Endpoints skip blank and non-RPC lines. client_rs does not panic.
- #[must_use] on builders and the lock guard; #[schemars] attributes
  and duplicate hooks are compile errors; raw identifiers; -32000 for
  local SDK errors; x-sausage-params only for methods with params.

sdk/tests/plugins/probe/Cargo.toml

@@ -5,3 +5,4 @@ edition = "2024"
 [workspace]
 [dependencies]
 sausage-plugin = { path = "../../.." }
+wstd = "0.6"

sdk/tests/plugins/probe/src/main.rs

@@ -70,7 +70,11 @@ impl Probe {
     }
     /// Return a deliberate error.
     fn fail(&self) -> Result<String> {
-        Err(sausage_plugin::Error::new(-32010, "deliberate"))
+        Err(sausage_plugin::Error {
+            code: -32010,
+            message: "deliberate".into(),
+            data: Some(json!({"diagnostic":"probe error details"})),
+        })
     }
     /// Exercise calls, notifications, and advisory locks.
     async fn exercise(&self, ctx: &Ctx) -> Result<String> {
@@ -119,6 +123,19 @@ impl Probe {
         });
         Ok("started".into())
     }
+    /// Keep a pollable alive for one hour.
+    fn sleepy(&self) -> Result<()> {
+        sausage_plugin::background(async {
+            wstd::task::sleep(wstd::time::Duration::from_secs(3600)).await;
+        });
+        Ok(())
+    }
+    /// Exit after the response despite a pending background pollable.
+    fn sleepy_exit(&self, ctx: &Ctx) -> Result<()> {
+        self.sleepy()?;
+        ctx.exit_after_response();
+        Ok(())
+    }
     /// Save an EOF marker.
     #[sausage_plugin::on_stop]
     async fn stop(&self) {