fix(agents): a startedAt too large for a float counts as live (Issue #31, review)

json.load reads a huge integer as an int, and math.isfinite raised
OverflowError on it outside any try: conversation_in_use and purge_artifacts
(which must never raise) threw, where 6f4f40a had taken it as live. The
conversion to float now comes first, and an overflow means live.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_issue31_purge_residual.py

@@ -472,7 +472,8 @@ def test_liveness_reads_lstart_as_utc(tmp_path, monkeypatch, live):
 
 
 @pytest.mark.parametrize("started", ["NaN", float("nan"), float("inf"), float("-inf"), "inf", "1760000000000",
-                                     "soon", True, False, None, [1], "<missing>"])
+                                     "soon", True, False, None, [1], pytest.param(10 ** 400, id="10**400"),
+                                     "<missing>"])
 def test_an_unusable_started_at_means_live(live, started):
     from lib_py.agents.adapters import claude as claude_mod
     data = {} if started == "<missing>" else {"startedAt": started}
@@ -480,10 +481,12 @@ def test_an_unusable_started_at_means_live(live, started):
     assert claude_mod._session_file_live(data, _dead_pid()) is False, "a dead pid holds nothing"
 
 
-@pytest.mark.parametrize("raw", ["NaN", '"NaN"', "Infinity", "true"])
+@pytest.mark.parametrize("raw", ["NaN", '"NaN"', "Infinity", "true",
+                                 pytest.param("1" + "0" * 400, id="10**400")])
 def test_a_live_holder_with_an_unusable_started_at_blocks_the_purge(home, tmp_path, conv, live, raw):
     """json.load turns a bare NaN or Infinity into a float that every
-    comparison fails, which once made a live holder look stale."""
+    comparison fails, which once made a live holder look stale, and a huge
+    integer into an int no float can hold."""
     ws = tmp_path / "ws"
     ws.mkdir()
     sfile, key = _session_file(home / ".claude", live.pid, conv)