test(02-04): add failing translation contract tests

- tests/test_translate.py (mt + slow markers): warm-up under 5 s and idempotent, six
contract sentences asserted on keywords with per-call ms printed, pieces end with </s>,
empty/whitespace raise ValueError, 200-char line bounded by max_decoding_length,
model_bytes matches disk, and no transformers/torch in sys.modules after warm-up
- Fails at collection: japanese_avatar.nlp.translate does not exist yet

tests/test_translate.py

@@ -0,0 +1,101 @@
+"""JPN-04 contract for the CPU translator: six sentences, no transformers, warm in under 5 s.
+
+Marked ``mt`` (and ``slow``) because it loads the CTranslate2 model (~0.3 s, +100 MB), which
+keeps it out of the quick loop; plan 02-05 runs the whole loop and records its wall time. Every
+assertion here is a keyword, never an exact string - the model is pinned, but a beam search is
+not a contract on wording. Per-call milliseconds are printed for the record and NOT bounded:
+research measured 16-129 ms locally at two threads, and the number that counts is the deployed
+one, which plan 02-10 measures on the Space.
+"""
+
+from __future__ import annotations
+
+import sys
+import time
+
+import pytest
+
+from japanese_avatar.nlp import translate as mt
+
+pytestmark = [pytest.mark.mt, pytest.mark.slow]
+
+# Japanese -> substrings of which at least one must appear (case-insensitive). Research § Q4
+# measured the full outputs; the keywords are the parts a paraphrase cannot drop.
+CONTRACT = [
+    ("こんにちは。", ("hello",)),
+    ("駅はどこですか。", ("station",)),
+    ("日本語を勉強しています。", ("japanese",)),
+    (
+        "今日はいい天気ですから、公園を散歩してから、買い物に行きました。",
+        ("park", "shopping", "weather"),
+    ),
+    ("日本語を練習しましょう。", ("japanese", "practice")),
+    ("昨日何をしましたか。", ("yesterday",)),
+]
+
+WARMUP_FIRST_MAX_S = 5.0  # research: 0.25-0.58 s locally; the Space CPU is 3-6x slower
+WARMUP_REPEAT_MAX_S = 0.01
+
+
+@pytest.fixture(scope="module")
+def warm() -> tuple[float, float]:
+    """Load the translator once per module; returns (first, second) warm-up seconds."""
+    first = mt.warmup()
+    second = mt.warmup()
+    print(f"\ntranslator warm-up {first:.3f} s, repeat {second:.4f} s")
+    return first, second
+
+
+def test_warmup_is_fast_and_idempotent(warm):
+    first, second = warm
+    assert first < WARMUP_FIRST_MAX_S, f"first warm-up took {first:.2f} s"
+    assert second < WARMUP_REPEAT_MAX_S, f"repeat warm-up took {second:.4f} s; not cached"
+    assert mt.get_translator() is mt.get_translator()
+
+
+@pytest.mark.parametrize(("text", "keywords"), CONTRACT, ids=[t for t, _ in CONTRACT])
+def test_contract_sentences(warm, text, keywords):
+    started = time.perf_counter()
+    out = mt.translate(text)
+    ms = (time.perf_counter() - started) * 1000.0
+    print(f"\n{text} -> {out!r} ({ms:.0f} ms)")
+    assert isinstance(out, str) and out.strip(), f"empty translation for {text!r}"
+    assert any(k in out.lower() for k in keywords), f"{out!r} contains none of {keywords}"
+
+
+def test_pieces_end_with_eos(warm):
+    pieces = mt.encode_pieces("こんにちは")
+    assert pieces[-1] == "</s>"
+    assert len(pieces) > 1
+    assert all(isinstance(p, str) for p in pieces)
+
+
+@pytest.mark.parametrize("text", ["", "   ", "\n\t"])
+def test_empty_and_whitespace_raise(warm, text):
+    with pytest.raises(ValueError):
+        mt.translate(text)
+
+
+def test_long_line_is_bounded(warm):
+    """A MAX_TEXT_CHARS-long line decodes within max_decoding_length and stays non-empty."""
+    text = ("日本語を勉強しています。" * 20)[: mt.MAX_TEXT_CHARS]
+    assert len(text) == mt.MAX_TEXT_CHARS
+    started = time.perf_counter()
+    out = mt.translate(text)
+    ms = (time.perf_counter() - started) * 1000.0
+    print(f"\n{len(text)}-char line -> {len(out)} chars ({ms:.0f} ms)")
+    assert out.strip()
+    _, _, target = mt.get_translator()
+    assert len(target.encode(out, out_type=str)) <= mt.DEFAULT_MAX_DECODING_LENGTH
+
+
+def test_model_bytes_is_the_committed_model(warm):
+    assert mt.model_bytes() == (mt.MODEL_DIR / "model.bin").stat().st_size
+    assert mt.model_bytes() > 70_000_000
+
+
+def test_no_transformers_or_torch_imported(warm):
+    """The Space's translation path is ctranslate2 + sentencepiece only (research § Q4)."""
+    assert "transformers" not in sys.modules
+    assert "torch" not in sys.modules
+    assert "ctranslate2" in sys.modules and "sentencepiece" in sys.modules