core/tools: add get_truncated tool for chained resume

tests/test_tools/test_get_truncated.py

@@ -0,0 +1,62 @@
+from __future__ import annotations
+
+import importlib
+from pathlib import Path
+
+from plyngent.tools.truncate_token import TruncateToken, decode_truncate_token, encode_truncate_token
+from tests.test_tools.helpers import call_async
+
+get_truncated_module = importlib.import_module("plyngent.tools.file.get_truncated")
+get_truncated = get_truncated_module.get_truncated
+
+
+def _extract_token(out: str) -> str | None:
+    for line in out.splitlines():
+        if "TRUNCATE_TOKEN:" in line:
+            return line.split("[TRUNCATE_TOKEN: ", 1)[1].rstrip("]")
+    return None
+
+
+async def test_get_truncated_invalid_token() -> None:
+    out = await call_async(get_truncated, "garbage-token")
+    assert out.startswith("error:")
+    assert "invalid" in out
+
+
+async def test_get_truncated_file_chains_without_gap(workspace: Path) -> None:
+    (workspace / "big.txt").write_text("a" * 1000, encoding="utf-8")
+    token = encode_truncate_token(TruncateToken(kind="file", location="big.txt", offset=0, limit=200))
+    total_read = 0
+    for _ in range(20):
+        out = await call_async(get_truncated, token, max_chars=200)
+        content, _, _ = out.partition("\n...[")
+        assert content == "a" * len(content)
+        total_read += len(content)
+        next_token = _extract_token(out)
+        if next_token is None:
+            break
+        parsed = decode_truncate_token(next_token)
+        assert parsed is not None
+        assert parsed.offset == total_read  # no gap, no overlap
+        token = next_token
+    assert total_read == 1000
+
+
+async def test_get_truncated_file_missing(workspace: Path) -> None:
+    token = encode_truncate_token(TruncateToken(kind="file", location="nope.txt", offset=0, limit=200))
+    out = await call_async(get_truncated, token, max_chars=200)
+    assert out.startswith("error:")
+
+
+async def test_get_truncated_http_passthrough(monkeypatch) -> None:
+    calls: list[tuple[str, int, int]] = []
+
+    async def fake_fetch(url: str, *, offset: int, max_chars: int) -> str:
+        calls.append((url, offset, max_chars))
+        return f"body-from-{offset}"
+
+    monkeypatch.setattr(get_truncated_module.fetch, "handler", fake_fetch)
+    token = encode_truncate_token(TruncateToken(kind="http", location="https://example.com/x", offset=500, limit=400))
+    out = await call_async(get_truncated, token, max_chars=200)
+    assert out == "body-from-500"
+    assert calls == [("https://example.com/x", 500, 400)]