fix: duplicate rendering of instructions in annotatable xblock (#36445)

In the Annotatable XBlock, the <instructions> element was appearing
twice in the student view:

* Once in "annotatable-instructions" (where it should be).
* Again in "annotatable-content" (where annotations and other content are rendered).

The _render_content method processed and rendered the entire XML
data, including <instructions>, without removing it. The
_extract_instructions method, which is responsible for removing
<instructions>, was not called in _render_content, leading to
duplication.

This fix will:

* Prevents duplicate instructions in the student view.
* Maintains the expected behavior of showing instructions only in
  "annotatable-instructions".
* No impact on existing annotation functionality.

xmodule/tests/test_annotatable_block.py

@@ -134,3 +134,11 @@ class AnnotatableBlockTestCase(unittest.TestCase):  # lint-amnesty, pylint: disa
         xmltree = etree.fromstring('<annotatable>foo</annotatable>')
         actual = self.annotatable._extract_instructions(xmltree)  # lint-amnesty, pylint: disable=protected-access
         assert actual is None
+
+    def test_instruction_removal(self):
+        xmltree = etree.fromstring(self.sample_xml)
+        instructions = self.annotatable._extract_instructions(xmltree)  # pylint: disable=protected-access
+
+        assert instructions is not None
+        assert "Read the text." in instructions
+        assert xmltree.find("instructions") is None