docs,test: fix two stale prose sites — errors.md recap bullet and the probe test block header

The recap bullet still taught the pre-branding 'not instanceof' rule the same
page retracts above it, and the test block header still described the rejected
legacy-fallback classification its own tests disprove.

Author: felixweinberger

docs/servers/errors.md

@@ -205,5 +205,5 @@ Match these by `code` and `data` shape when peers may run pre-brand SDK copies o
 - A tool handler that throws produces the same `isError: true` result; the exception's `message` becomes the `content` text.
 - A tool handler cannot produce a protocol error — only `UrlElicitationRequiredError` escapes.
 - `ProtocolError` and its subclasses, thrown from resource, prompt, and completion callbacks, become JSON-RPC error responses the model never sees.
-- `ResourceNotFoundError` and the other subclasses pick the code and pack structured `data`; match them by `code` and `data`, not `instanceof`.
+- `ResourceNotFoundError` and the other subclasses pick the code and pack structured `data`; match them by `code` and `data` — or, on brand-aware releases, by `instanceof`.
 - The table above lists every `ProtocolErrorCode` member.

packages/client/test/client/versionNegotiation.test.ts

@@ -709,8 +709,9 @@ describe('era scope discipline', () => {
 });
 
 /* ------------------------------------------------------------------------- *
- * Probe send-error classification: auth-gated servers take the legacy
- * fallback; other send failures stay typed negotiation errors.
+ * Probe send-error classification: auth-gated servers propagate the
+ * UnauthorizedError unchanged (finishAuth() + reconnect probes again); other
+ * send failures stay typed negotiation errors.
  * ------------------------------------------------------------------------- */
 
 describe('probe send-error classification', () => {