[bugfix] Force HTTP/1.1 in AbstractHttpTest and fix mock expectations in TxnTest

Two fixes bundled here:

1. AbstractHttpTest.newHttpClient(): add .version(HTTP_1_1) to prevent RST_STREAM
   hangs. GetParameterTest (and 10 other RESTTest subclasses) were silently stalling
   under HTTP/2, causing the 45-minute CI unit-test timeout to trigger. Fixing the
   shared factory covers all subclasses in one place.

2. TransactionManagerTestHelper: add expect(isShuttingDownOrDown()).andReturn(false)
   to the EasyMock setup. Our TransactionManager.processSystemTasks() change added a
   pool.isShuttingDownOrDown() guard; the strict mock had no expectation for that call,
   causing TxnTest and ReusableTxnTest to fail with "Unexpected method call".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: duncdrum

exist-core/src/test/java/org/exist/http/AbstractHttpTest.java

@@ -103,7 +103,9 @@ public static String basicAuthorizationHeader(final String user, final String pa
      * @return a new {@link HttpClient}.
      */
     public static HttpClient newHttpClient() {
+        // Force HTTP/1.1: RST_STREAM is an HTTP/2 frame; using HTTP/1.1 eliminates that failure mode.
         return HttpClient.newBuilder()
+                .version(HttpClient.Version.HTTP_1_1)
                 .followRedirects(HttpClient.Redirect.NORMAL)
                 .build();
     }

exist-core/src/test/java/org/exist/storage/txn/TransactionManagerTestHelper.java

@@ -69,6 +69,7 @@ protected TransactionManager createTestableTransactionManager(final boolean expe
         expect(mockBrokerPool.get(Optional.of(mockSystemSubject))).andReturn(mockBroker).anyTimes();
         expect(mockBrokerPool.getSecurityManager()).andReturn(mockSecurityManager).anyTimes();
         expect(mockSecurityManager.getSystemSubject()).andReturn(mockSystemSubject).anyTimes();
+        expect(mockBrokerPool.isShuttingDownOrDown()).andReturn(false).anyTimes();
 
         final JournalManager mockJournalManager = createMock(JournalManager.class);
         final SystemTaskManager mockTaskManager = createMock(SystemTaskManager.class);