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/params.stderr

@@ -12,7 +12,7 @@ help: the trait `Params` is implemented for `sausage_plugin::Empty`
 note: required by a bound in `sausage_plugin::method`
  --> src/discovery.rs
   |
-  | pub fn method<P: Params, R: SchemaType>(doc: &mut Value, name: &str, docs: &str) {
+  | pub fn method<P: Params, R: SchemaType>(doc: &mut Discovery, name: &str, docs: &str) {
   |                  ^^^^^^ required by this bound in `method`
 
 error[E0277]: RPC params must be a #[schema] struct with named fields