test(router): pin the cut point and ttl of a prefix with turns too deep to serialize

/live-pr-risk found two mutants of the round 16 cut that no test killed. One cut at the last turn the router cannot turn
into JSON instead of the first, and one took the pin ttl from every readable turn instead of the cut prefix. These tests
check that a pin written past two such turns lands on the mark before the first, and that a pin cut before such a turn
lasts only as long as the marks it kept

tests/unit/router_utils/test_prompt_caching_cache.py

@@ -426,6 +426,57 @@ def test_a_marked_turn_too_deep_to_serialize_keeps_the_pin_of_the_mark_before_it
     assert cache.get_model_id(messages=first_turn, tools=None) == {"model_id": "deployment-2"}
 
 
+def _deep_marked_question(question: str) -> AllMessageValues:
+    return cast(
+        AllMessageValues,
+        {"role": "user", "content": question, "cache_control": {"type": "ephemeral"}, "x": _DEEP_VALUE},
+    )
+
+
+def test_a_pin_written_past_two_turns_too_deep_to_serialize_lands_on_the_mark_before_the_first() -> None:
+    cache: Final = _pin_cache(_ManualClock())
+    first_turn: Final = _system_block_messages({"type": "ephemeral"})
+    messages: Final = [
+        *first_turn,
+        _deep_marked_question("Find the termination clause."),
+        cast(
+            AllMessageValues,
+            {"role": "user", "content": "Now the renewal clause.", "cache_control": {"type": "ephemeral"}},
+        ),
+        _deep_marked_question("And the governing law."),
+    ]
+
+    cache.add_model_id(model_id="deployment-1", messages=first_turn, tools=None)
+    cache.add_model_id(model_id="deployment-2", messages=messages, tools=None)
+
+    assert cache.get_model_id(messages=first_turn, tools=None) == {"model_id": "deployment-2"}
+
+
+def test_a_pin_cut_before_a_turn_too_deep_to_serialize_lasts_only_as_long_as_the_marks_it_kept() -> None:
+    clock: Final = _ManualClock()
+    cache: Final = _pin_cache(clock)
+    first_turn: Final = _system_block_messages({"type": "ephemeral"})
+    messages: Final = cast(
+        list[AllMessageValues],
+        [
+            *first_turn,
+            {"role": "assistant", "content": None, "tool_calls": [{**_tool_call("call_1"), "x": _DEEP_VALUE}]},
+            {"role": "tool", "tool_call_id": "call_1", "content": "Section 9."},
+            {
+                "role": "user",
+                "content": "Find the renewal clause.",
+                "cache_control": {"type": "ephemeral", "ttl": "1h"},
+            },
+        ],
+    )
+
+    cache.add_model_id(model_id="deployment-1", messages=messages, tools=None, honors_one_hour_ttl=True)
+
+    assert cache.get_model_id(messages=first_turn, tools=None) == {"model_id": "deployment-1"}
+    clock.advance(PROMPT_CACHE_PIN_TTL_SECONDS + 1)
+    assert cache.get_model_id(messages=first_turn, tools=None) is None
+
+
 async def _first_non_none(read):
     while True:
         value = await read()