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/hooks.stderr

@@ -0,0 +1,19 @@
+error: only one on_stop method is allowed
+ --> tests/ui/hooks.rs:8:5
+  |
+8 |     /// Second stop hook.
+  |     ^^^^^^^^^^^^^^^^^^^^^
+
+error: only one on_retired method is allowed
+  --> tests/ui/hooks.rs:18:5
+   |
+18 |     /// Second retired hook.
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused imports: `Result`, `on_retired`, and `on_stop`
+ --> tests/ui/hooks.rs:1:27
+  |
+1 | use sausage_plugin::{rpc, on_stop, on_retired, Result};
+  |                           ^^^^^^^  ^^^^^^^^^^  ^^^^^^
+  |
+  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default