tests(internal): add direct unit tests for retry_extended

why: Module had no direct tests, only indirect coverage via waiter mocking
what:
- Test immediate success (returns True on first call)
- Test success after multiple retries
- Test timeout with raises=True (raises WaitTimeout)
- Test timeout with raises=False (returns False, WaitTimeout)

Author: tony

tests/_internal/test_retry_extended.py

@@ -0,0 +1,69 @@
+"""Tests for retry_until_extended function."""
+
+from __future__ import annotations
+
+import pytest
+
+from libtmux._internal.retry_extended import retry_until_extended
+from libtmux.exc import WaitTimeout
+
+
+def test_retry_success_immediate() -> None:
+    """Test function returns True immediately on first call."""
+    call_count = 0
+
+    def always_true() -> bool:
+        nonlocal call_count
+        call_count += 1
+        return True
+
+    success, exception = retry_until_extended(always_true, seconds=1.0)
+
+    assert success is True
+    assert exception is None
+    assert call_count == 1
+
+
+def test_retry_success_after_attempts() -> None:
+    """Test function succeeds after a few retries."""
+    call_count = 0
+
+    def succeeds_on_third() -> bool:
+        nonlocal call_count
+        call_count += 1
+        return call_count >= 3
+
+    success, exception = retry_until_extended(
+        succeeds_on_third,
+        seconds=2.0,
+        interval=0.01,
+    )
+
+    assert success is True
+    assert exception is None
+    assert call_count == 3
+
+
+def test_retry_timeout_raises() -> None:
+    """Test timeout raises WaitTimeout when raises=True."""
+    with pytest.raises(WaitTimeout, match="Timed out after"):
+        retry_until_extended(
+            lambda: False,
+            seconds=0.05,
+            interval=0.01,
+            raises=True,
+        )
+
+
+def test_retry_timeout_no_raise() -> None:
+    """Test timeout returns (False, WaitTimeout) when raises=False."""
+    success, exception = retry_until_extended(
+        lambda: False,
+        seconds=0.05,
+        interval=0.01,
+        raises=False,
+    )
+
+    assert success is False
+    assert isinstance(exception, WaitTimeout)
+    assert "Timed out after" in str(exception)