[lldb-dap] Address a unit test race condition during initialization. (#167981)

During the initialization sequence in our tests the first 'threads'
response sould only be kept if the process is actually stopped,
otherwise we will have stale data.

In VSCode, during the debug session startup sequence immediately after
'configurationDone' a 'threads' request is made. This initial request is
to retrieve the main threads name and id so the UI can be populated.
However, in our tests we do not want to cache this value unless the
process is actually stopped. We do need to make this initial request
because lldb-dap is caching the initial thread list during
configurationDone before the process is resumed. We need to make this
call to ensure the cached initial threads are purged.

I noticed this in a CI job for another review
(https://github.com/llvm/llvm-project/actions/runs/19348261989/job/55353961798)
where the tests incorrectly failed to fetch the threads prior to
validating the thread names.

Author: ashgti

lldb/packages/Python/lldbsuite/test/tools/lldb-dap/dap_server.py

@@ -191,6 +191,11 @@ class NotSupportedError(KeyError):
 
 
 class DebugCommunication(object):
+    @property
+    def is_stopped(self) -> bool:
+        """Returns True if the debuggee is stopped, otherwise False."""
+        return len(self.thread_stop_reasons) > 0 or self.exit_status is not None
+
     def __init__(
         self,
         recv: BinaryIO,
@@ -860,7 +865,17 @@ def request_configurationDone(self):
         response = self._send_recv(command_dict)
         if response:
             self.configuration_done_sent = True
+            stopped_on_entry = self.is_stopped
             self.request_threads()
+            if not stopped_on_entry:
+                # Drop the initial cached threads if we did not stop-on-entry.
+                # In VSCode, immediately following 'configurationDone', a
+                # 'threads' request is made to get the initial set of threads,
+                # specifically the main threads id and name.
+                # We issue the threads request to mimic this pattern but in our
+                # tests we don't want to cache the result unless the process is
+                # actually stopped.
+                self.threads = None
         return response
 
     def _process_stopped(self):