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/ui/schemars.rs

@@ -0,0 +1,9 @@
+use sausage_plugin::schema;
+/// A documented object.
+#[schema]
+struct Object {
+    /// A documented field.
+    #[schemars(with = "std::collections::HashMap<String, String>")]
+    value: String,
+}
+fn main() {}