fix(client): classify discover outcome at source, not at the error type (#1133)

`ClientLifecycleMode::Auto` only fell back from `server/discover` on
`-32601`, so legacy servers that reject the probe with other codes
(`-32600`, `-32602`, implementation-defined errors) failed to connect
even though `initialize` would have succeeded.

The previous attempt (indicates_legacy_server) classified the failure
after the fact by reverse-engineering the error type. This rewrite moves
the classification into `discover_startup` itself, where the full context
(request id, response correlation, transport state) is still available.

`discover_startup` now returns `DiscoverOutcome`: `Modern` on success,
`Legacy(error)` when the probe received a complete, correlated JSON-RPC
error whose code is not a modern-era rejection. Every other failure
becomes `Err`, so `Auto` simply matches the outcome — no methods on
`ClientInitializeError`, no downcast, no transport-specific types
leaking into the generic lifecycle layer.

Additional fixes that fall out naturally:

- Response correlation is now checked in `expect_response` for both
  success and error branches. Previously error responses skipped id
  correlation entirely. A new `UncorrelatedErrorResponse` variant
  surfaces responses that cannot be tied to the request.

- When both discover and the legacy fallback fail, a
  `LegacyFallbackFailed` compound error preserves both phases instead
  of discarding the discover error.

Fixes #1040.

crates/rmcp/tests/test_client_initialization.rs

@@ -125,11 +125,18 @@ async fn test_client_init_handles_jsonrpc_error() {
     });
 
     tokio::spawn(async move {
-        let _init_request = server.receive().await;
+        let request = server.receive().await;
+        // Echo the request's own id back on the error so it correlates: an
+        // uncorrelated id would surface as `UncorrelatedErrorResponse`
+        // instead of the `JsonRpcError` this test exercises.
+        let request_id = request
+            .and_then(|message| message.into_request())
+            .map(|(_, id)| id)
+            .expect("client sent an initialize request");
 
         let error_msg = ServerJsonRpcMessage::Error(JsonRpcError {
             jsonrpc: JsonRpcVersion2_0,
-            id: Some(RequestId::Number(1)),
+            id: Some(request_id),
             error: ErrorData {
                 code: ErrorCode(-32600),
                 message: Cow::Borrowed("Invalid Request"),