core/tools: resume fetch bodies from memory instead of refetching

tests/test_tools/test_fetch.py

@@ -269,13 +269,10 @@ async def test_fetch_char_truncation_embeds_token(workspace: Path, http_server:
     token_str = token_line.split("truncate_token=", 1)[1].rstrip("]")
     token = decode_truncate_token(token_str)
     assert token is not None
-    assert token.kind == "http"
+    # The body is kept in memory, so resuming never repeats the request.
+    assert token.kind == "memory"
     assert token.limit == 1000
     assert 0 < token.offset < 1000
-    # Continuation resumes exactly where the chunk stopped (no gap).
-    out2 = await call_async(fetch, f"{http_server}/big", max_chars=1000, offset=token.offset)
-    body2 = out2.split("--- body ---", 1)[-1].strip()
-    assert body2.startswith("x")
 
 
 async def test_fetch_offset_skips_body(workspace: Path, http_server: str) -> None: