test(compressor): drop source-string guardrail tests

The two TestSourceGuardrail tests asserted the presence of literal
strings ("#58753", "_user_survives") in context_compressor.py. Those
are change-detector tests that break on any refactor without catching a
real regression. The four behavioral tests in
TestCompressAlwaysKeepsAUserTurn already exercise the real compress()
path and fully cover the invariant (user turn survives, summary pinned
to user, no consecutive user roles, surviving tail user untouched).

tests/agent/test_compressor_zero_user_guard.py

@@ -178,29 +178,3 @@ class TestCompressAlwaysKeepsAUserTurn:
             m.get("content") for m in out if isinstance(m.get("content"), str)
         )
         assert "the latest live user question" in joined
-
-
-class TestSourceGuardrail:
-    @pytest.fixture
-    def source(self) -> str:
-        from pathlib import Path
-
-        return (
-            Path(__file__).resolve().parents[2]
-            / "agent"
-            / "context_compressor.py"
-        ).read_text(encoding="utf-8")
-
-    def test_zero_user_guard_present(self, source):
-        assert "#58753" in source
-        # The guard must consider messages OUTSIDE the (system-only) head
-        # case — i.e. it inspects the preserved tail for a surviving user.
-        assert "_user_survives" in source
-
-    def test_guard_wired_into_force_user_leading(self, source):
-        """The guard must feed ``_force_user_leading`` so the existing
-        flip logic cannot revert the summary back to assistant."""
-        idx_guard = source.find("_user_survives")
-        idx_force = source.rfind("_force_user_leading = True")
-        assert idx_guard >= 0 and idx_force >= 0
-        assert idx_force > idx_guard