feat(content): reject all-filler multi-token strings in reject_trivial

The single non-empty/non-placeholder gate caught a lone banned token
(wip) and below-floor strings, but multi-token soup made entirely of
placeholders (wip wip, tbd / na, todo todo todo) slipped through both
checks. Add an all-tokens-filler test that strips edge punctuation per
token and rejects when every meaningful token is banned — without
flagging real prose that merely contains a filler word (none of the
tests failed). Strengthens every content model + gateway anti-soup
guard that composes reject_trivial.

tests/unit/foundation/policy/content/test_validators.py

@@ -0,0 +1,59 @@
+"""Tests for the shared content validators (``reject_trivial``).
+
+``reject_trivial`` is the single non-empty / non-placeholder gate reused by
+every structured-content field validator AND the gateway anti-soup guards. It
+must reject: empty, too-short, a lone placeholder token, AND a string whose
+every whitespace token is a placeholder (``wip wip``, ``tbd / na``).
+"""
+
+from __future__ import annotations
+
+import pytest
+from roboco.foundation.policy.content.validators import reject_trivial
+
+
+def test_returns_trimmed_value_when_substantive() -> None:
+    assert reject_trivial("  real content here  ", field="x") == "real content here"
+
+
+@pytest.mark.parametrize("blank", ["", "   ", "\t\n"])
+def test_rejects_empty(blank: str) -> None:
+    with pytest.raises(ValueError, match="must not be empty"):
+        reject_trivial(blank, field="summary")
+
+
+def test_rejects_too_short() -> None:
+    with pytest.raises(ValueError, match="at least 10 characters"):
+        reject_trivial("short", field="summary", min_chars=10)
+
+
+@pytest.mark.parametrize(
+    "token", ["wip", "TBD", "asdf", "n/a", "none", "-", "...", "x"]
+)
+def test_rejects_lone_placeholder_token(token: str) -> None:
+    with pytest.raises(ValueError, match="placeholder"):
+        reject_trivial(token, field="summary")
+
+
+@pytest.mark.parametrize(
+    "soup",
+    ["wip wip", "tbd / na", "todo todo todo", "asdf asdf asdf asdf", "none none"],
+)
+def test_rejects_all_filler_token_string(soup: str) -> None:
+    # Every token is a placeholder — the whole string is soup even though it is
+    # neither a single banned token nor below the length floor.
+    with pytest.raises(ValueError, match="placeholder"):
+        reject_trivial(soup, field="summary")
+
+
+@pytest.mark.parametrize(
+    "ok",
+    [
+        "none of the tests failed",  # 'none' present but not all-filler
+        "fixed the x coordinate bug",  # 'x' present but not all-filler
+        "rate_limited",  # a real substitute reason
+        "LGTM, merging now",
+    ],
+)
+def test_accepts_real_text_containing_a_filler_word(ok: str) -> None:
+    assert reject_trivial(ok, field="reason", min_chars=3) == ok.strip()