fix(sessions): keep the interactive exclusion spelled as a literal

tests/test_issue2841_show_cron_sessions_toggle.py reads this call site as
SOURCE TEXT (`'("cron", "webhook", "kanban") if source_filter is None'`), so
routing it through the new BACKGROUND_CLI_SOURCES constant turned a
pre-existing test in the maintainer's own shard gate red for a
behaviour-neutral refactor.

Put the literal back at that one call site. BACKGROUND_CLI_SOURCES stays where
the kanban-chip fix actually needs it (the background-source check in
_interactive_row_project_id, and interactive_excluded for both recovery
passes). A new test builds the expected literal FROM the constant and asserts
it appears verbatim in api/models.py, so the two spellings cannot drift
without failing here rather than in an unrelated file.

tests/test_project_assigned_cli_session_limit.py

@@ -756,6 +756,28 @@ def test_unassigned_refill_runs_when_the_mixed_window_under_delivers(
 # ── Rebase guard: upstream's kanban pass shares the system-chip helper ────────
 
 
+def test_background_source_exclusion_literal_matches_the_constant():
+    """The interactive exclusion stays spelled as a tuple literal.
+
+    ``tests/test_issue2841_show_cron_sessions_toggle.py`` reads that call site as
+    SOURCE TEXT (``'("cron", "webhook", "kanban") if source_filter is None'``),
+    so replacing the literal with ``BACKGROUND_CLI_SOURCES`` turned a
+    pre-existing test red. The literal is therefore kept, and this test pins it
+    to the constant the new code paths use so the two cannot drift apart.
+    """
+    from pathlib import Path
+
+    source = Path(models.__file__).read_text(encoding="utf-8")
+    literal = ", ".join(f'"{name}"' for name in models.BACKGROUND_CLI_SOURCES)
+    assert f"exclude_sources=({literal}) if source_filter is None else None" in source, (
+        "the interactive exclude_sources literal must list exactly "
+        f"BACKGROUND_CLI_SOURCES ({models.BACKGROUND_CLI_SOURCES!r})"
+    )
+    assert '("cron", "webhook", "kanban") if source_filter is None' in source, (
+        "test_issue2841_show_cron_sessions_toggle.py pins this exact substring"
+    )
+
+
 def test_kanban_second_pass_still_projects_rows(fake_hermes_home, tmp_path):
     """The system-chip helper keeps the (sid, source) signature kanban calls.