fix(tools): search_and_fetch のページングトークン破損バグを修正

## 背景

search_and_fetch の next_token / continue_from によるページング継続が
PDF 形式で間欠的に失敗するバグが報告された。発生エラー:
  1. payload is not valid JSON: Expecting ':' delimiter
  2. 'utf-8' codec can't decode byte 0xda in position 278

1ページ目の取得自体は成功し、next_token 返却まで正常だが、
next_token を continue_from に渡した2ページ目でエラーが発生。
txt/md/csv/json では安定動作し、PDF で間欠的・再現性なし。

## 根本原因

コード精査の結果、報告の「非UTF-8バイト混入」仮説に対し、より有力な
主因として「トークン巨大化による末尾欠損」を特定:

- _render_file_page() は matched 配列（title/href/body 全部入り）を
  トークンに埋め込んでいたが、レンダリングで使うのは title と href
  のみ。body（DuckDuckGo のスニペット、PDF 論文要約で数KB）は未使用
- 候補複数件でトークンが数KB〜十数KBに膨張
- LLM が continue_from にコピーする際、コンテキスト長や
  _trim_tool_result の切り詰めでトークン末尾が欠損
- base64 デコード結果が不正になり JSONDecodeError（JSON途中切断）
  と UnicodeDecodeError（マルチバイト境界切断）の両エラーが発生
- PDF で間欠的・再現性なしは、検索結果セットごとにスニペット長が
  変動しトークンサイズが破損閾値を前後するためと説明がつく

補助要因: make_token() の ensure_ascii=False が、末尾欠損時に
マルチバイト文字境界で切れる確率を上げていた。

## 修正内容（案A: ユーザー承認済み）

1. pagination.py の make_token() を ensure_ascii=True 化
   - トークンを純ASCII化し、末尾欠損でも UnicodeDecodeError を防止
   - 非ASCII文字は \uXXXX エスケープ（6バイト固定）で安定化

2. fetch.py に _slim_matched() ヘルパーを追加
   - matched から body を除外し title+href のみ埋め込み
   - 3箇所の make_token 呼び出し（fetch失敗時・ファイル内継続時・
     次ファイル移行時）を matched=_slim_matched(matched) に変更
   - トークンサイズを数KB〜十数KB → 数百バイトに削減

3. トークン形式バージョン v は 1維持（構造不変・後方互換）
   - 旧トークン（body入り・ensure_ascii=False）も parse_token で
     正常デコード可能
   - 再検索回避機能も維持

## テスト

tests/test_tools.py に新規テスト5件を追加:
- test_make_token_ensure_ascii_true
- test_search_and_fetch_token_excludes_body
- test_search_and_fetch_token_size_reduced
- test_search_and_fetch_continue_with_slim_token
- test_search_and_fetch_pagination_with_nonascii_title

## 検証結果

- pytest: 113 passed (新規5件含む、既存回帰なし)
- ruff check src/ tests/: All checks passed
- mypy src/: Success: no issues found in 30 source files

## ドキュメント

docs/tools-schema.md に実装ベースの差分を反映:
- make_token() の説明に ensure_ascii=True 化の旨を追記
- search_and_fetch 処理フローの注記に _slim_matched() による
  body 除外とトークンサイズ削減の旨を追記
- トークンペイロード構造の matched 例から body を削除

1 files changed, 117 insertions, 0 deletions

diff --git a/tests/test_tools.py b/tests/test_tools.py
index 67c66ef..a94ea52 100644
--- a/tests/test_tools.py
+++ b/tests/test_tools.py
@@ -354,6 +354,123 @@ def test_search_and_fetch_pagination_across_files(monkeypatch):
     assert seen_titles == ["File1", "File2"]
 
 
+def test_make_token_ensure_ascii_true():
+    """Tokens are pure ASCII even with non-ASCII content (ensure_ascii=True)."""
+    token = make_token("scrape_url", "https://example.com/日本語", 0, title="論文–要約")
+    # base64url alphabet is pure ASCII; any non-ASCII byte would break this.
+    assert all(ord(c) < 128 for c in token)
+    payload = parse_token(token, "scrape_url")
+    assert payload["url"] == "https://example.com/日本語"
+    assert payload["title"] == "論文–要約"
+
+
+def test_search_and_fetch_token_excludes_body(monkeypatch):
+    """The continuation token must not embed the search snippet (body)."""
+    from ollama_web.tools import fetch as fetch_mod
+
+    matched = [
+        {
+            "title": "File1",
+            "href": "https://example.com/f1.txt",
+            "body": "X" * 5000,  # large snippet that would bloat the token
+        }
+    ]
+    monkeypatch.setattr(fetch_mod, "web_search", lambda q, max_results=None: matched)
+    monkeypatch.setattr(
+        fetch_mod,
+        "fetch_public_bytes",
+        lambda url, timeout=None: (b"A" * 200, "", "text/plain"),
+    )
+    result = fetch_mod.search_and_fetch("query", "txt", max_chars=50)
+    _, _, next_token = parse_paged_result(result)
+    assert next_token is not None
+    payload = parse_token(next_token, "search_and_fetch")
+    embedded = payload.get("matched", [])
+    assert embedded, "matched list should be embedded for resume"
+    for entry in embedded:
+        assert "body" not in entry, "body must be stripped from the token"
+        assert "title" in entry and "href" in entry
+
+
+def test_search_and_fetch_token_size_reduced(monkeypatch):
+    """A large body snippet must not inflate the token."""
+    from ollama_web.tools import fetch as fetch_mod
+
+    matched = [
+        {
+            "title": "File1",
+            "href": "https://example.com/f1.txt",
+            "body": "B" * 10_000,
+        }
+    ]
+    monkeypatch.setattr(fetch_mod, "web_search", lambda q, max_results=None: matched)
+    monkeypatch.setattr(
+        fetch_mod,
+        "fetch_public_bytes",
+        lambda url, timeout=None: (b"A" * 200, "", "text/plain"),
+    )
+    result = fetch_mod.search_and_fetch("query", "txt", max_chars=50)
+    _, _, next_token = parse_paged_result(result)
+    assert next_token is not None
+    # Token should be far smaller than the 10KB body it would otherwise carry.
+    assert len(next_token) < 1000
+
+
+def test_search_and_fetch_continue_with_slim_token(monkeypatch):
+    """Resuming with a slim (body-less) token walks files correctly."""
+    from ollama_web.tools import fetch as fetch_mod
+
+    matched = [
+        {"title": "File1", "href": "https://example.com/f1.txt", "body": "ignored1"},
+        {"title": "File2", "href": "https://example.com/f2.txt", "body": "ignored2"},
+    ]
+    monkeypatch.setattr(fetch_mod, "web_search", lambda q, max_results=None: matched)
+    contents = {
+        "https://example.com/f1.txt": b"FILE1 " + b"A" * 200,
+        "https://example.com/f2.txt": b"FILE2 " + b"B" * 200,
+    }
+    monkeypatch.setattr(
+        fetch_mod,
+        "fetch_public_bytes",
+        lambda url, timeout=None: (contents[url], "", "text/plain"),
+    )
+
+    seen = []
+    token = None
+    for _ in range(20):
+        result = fetch_mod.search_and_fetch("query", "txt", max_chars=50, continue_from=token)
+        body_text, has_more, token = parse_paged_result(result)
+        if "File1" in body_text and "File1" not in seen:
+            seen.append("File1")
+        if "File2" in body_text and "File2" not in seen:
+            seen.append("File2")
+        if not has_more:
+            break
+    assert seen == ["File1", "File2"]
+
+
+def test_search_and_fetch_pagination_with_nonascii_title(monkeypatch):
+    """Non-ASCII titles/URLs survive a full pagination round-trip."""
+    from ollama_web.tools import fetch as fetch_mod
+
+    matched = [
+        {"title": "日本語–タイトル", "href": "https://example.com/論文.txt", "body": ""},
+    ]
+    monkeypatch.setattr(fetch_mod, "web_search", lambda q, max_results=None: matched)
+    monkeypatch.setattr(
+        fetch_mod,
+        "fetch_public_bytes",
+        lambda url, timeout=None: (b"A" * 200, "", "text/plain"),
+    )
+    result = fetch_mod.search_and_fetch("query", "txt", max_chars=50)
+    body_text, has_more, next_token = parse_paged_result(result)
+    assert "日本語–タイトル" in body_text
+    assert has_more is True
+    # Resuming must not raise even though the token carries non-ASCII fields.
+    result2 = fetch_mod.search_and_fetch("query", "txt", max_chars=50, continue_from=next_token)
+    assert isinstance(result2, str)
+
+
 # ---------------------------------------------------------------------------
 # pagination helper tests
 # ---------------------------------------------------------------------------