[doc] Update known-build-issues docs and pom comment

AGENTS.md: MoveResourceTest no longer hangs after the HTTP/1.1 fix — remove it
from the warning. Document the two permanently excluded tests (FragmentsTest,
ConcurrencyTest) with their root causes.

exist-core/pom.xml: FragmentsTest comment previously claimed the test passes
locally in ~54s. Confirmed to hang locally as well (killed after 12 min);
root cause is not fully resolved by the isShuttingDownOrDown fix alone.

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

Author: duncdrum

AGENTS.md

@@ -98,8 +98,9 @@ docker run -d --name existdb -p 8080:8080 -p 8443:8443 existdb/existdb:local
 
 ### Known build issues
 
-- Full test suite can hang on flaky infrastructure tests (`MoveResourceTest`, `RenameCollectionTest`). Check with `jstack` and kill if stuck >15 min.
+- Full test suite can hang on flaky infrastructure tests (`RenameCollectionTest`). Check with `jstack` and kill if stuck >15 min.
 - `RenameCollectionTest` "Connection refused" failures are pre-existing and unrelated to XQuery changes.
+- `org.exist.xmldb.concurrent.FragmentsTest` and `org.exist.collections.ConcurrencyTest` are excluded from the surefire run (see `exist-core/pom.xml`). `FragmentsTest` hangs during BrokerPool shutdown even locally; `ConcurrencyTest` can deadlock under concurrent collection access.
 
 ## Parser (ANTLR 2)
 

exist-core/pom.xml

@@ -1216,8 +1216,8 @@ The BaseX Team. The original license statement is also included below.]]></pream
                                  see https://github.com/eXist-db/exist/issues/3685 -->
                         <exclude>org.exist.collections.ConcurrencyTest</exclude>
 
-                        <!-- Passes locally (~54s) but hangs on CI during BrokerPool shutdown,
-                                 causing the entire unit test step to time out.
+                        <!-- Hangs both locally and on CI during BrokerPool shutdown;
+                                 root cause not fully resolved (isShuttingDownOrDown fix was insufficient).
                                  see https://github.com/eXist-db/exist/pull/6153 -->
                         <exclude>org.exist.xmldb.concurrent.FragmentsTest</exclude>
                     </excludes>

exist-core/src/test/java/org/exist/xquery/XQueryUpdateTest.java

@@ -515,6 +515,9 @@ public void removeTestData() throws EXistException, PermissionDeniedException, I
     }
 
 
+    // Precondition: caller must hold an active DBBroker on the current thread (e.g. via withBroker).
+    // Txn.close() calls pool.getBroker() to remove the transaction; without an active broker on the
+    // thread it may draw a different instance and throw IllegalStateException.
     private void store(DBBroker broker, String docName, String data) throws PermissionDeniedException, EXistException, SAXException, LockException, IOException {
         Collection root;
         final BrokerPool pool = existEmbeddedServer.getBrokerPool();