fix(preview): fetch ceiling tracks the widest kind cap, not a flat 10 MB

Review feedback (PR #800): the URL lane hard-capped fetched bodies at
10 MB before kind resolution, making the 32 MiB pdf cap unreachable for
URL targets while path targets honored it. The flat pre-check is gone;
the guarded fetch's max_bytes now tracks max(PREVIEW_SIZE_CAPS.values())
- mirroring the path lane's stat pre-check - and the per-kind caps after
resolution stay authoritative.

Also drops a redundant function-local asyncio import in test_console.py.

tests/test_console.py

@@ -2699,7 +2699,6 @@ class TestProxyGetHeaderPassThrough:
     finding, preview-pane branch)."""
 
     def test_security_headers_forwarded(self, monkeypatch):
-        import asyncio
         from types import SimpleNamespace
         from unittest.mock import MagicMock
 

tests/test_open_preview_tool.py

@@ -183,6 +183,29 @@ class TestExecOpenPreview:
         assert msg.startswith("Error:")
         assert "too large" in msg
 
+    def test_url_pdf_over_10mb_previews_to_kind_cap(self, monkeypatch):
+        # Review finding (PR #800): a flat 10 MB URL pre-check rejected PDFs
+        # the 32 MiB pdf kind cap allows — the fetch ceiling must track the
+        # widest kind cap and leave the per-kind caps as the authority.
+        from turnstone.core.preview import PREVIEW_SIZE_CAPS
+
+        s = _make_session()
+        body = b"%PDF-1.7\n" + b"a" * (12 * 1024 * 1024)
+        seen = {}
+
+        def _capture(url, **kw):
+            seen.update(kw)
+            return _fake_response(url, body, "application/pdf")
+
+        monkeypatch.setattr("turnstone.core.session.fetch_with_ssrf_guard", _capture)
+        item = s._prepare_open_preview("c1", {"target": "https://acme.com/report.pdf"})
+        _, msg = s._exec_open_preview(item)
+        assert not msg.startswith("Error:")
+        descriptor, _ = s._tool_previews["c1"]
+        assert descriptor["kind"] == "pdf"
+        assert descriptor["size"] == len(body)
+        assert seen["max_bytes"] == max(PREVIEW_SIZE_CAPS.values())
+
     def test_path_image(self, tmp_path):
         s = _make_session()
         p = tmp_path / "chart.png"

turnstone/core/session.py

@@ -16159,11 +16159,14 @@ class ChatSession:
             try:
                 # Every redirect hop is SSRF-screened BEFORE its request goes
                 # out — the pre-approval screen covers only the URL the model
-                # named, not where it 302s.
+                # named, not where it 302s.  The fetch ceiling tracks the most
+                # permissive kind cap (like the path lane's stat pre-check);
+                # the per-kind cap after resolution is the authority.
                 resp = fetch_with_ssrf_guard(
                     url,
                     timeout=self.tool_timeout,
                     allow_private_origin=item.get("allow_private_origin", False),
+                    max_bytes=max(PREVIEW_SIZE_CAPS.values()),
                 )
                 resp.raise_for_status()
             except httpx.HTTPStatusError as e:
@@ -16177,10 +16180,6 @@ class ChatSession:
             if resp.url.username or resp.url.password:
                 final_url = str(resp.url.copy_with(username=None, password=None))
             body = resp.content
-            if len(body) > 10 * 1024 * 1024:
-                return _fail(
-                    f"Error: response too large to preview ({len(body):,} bytes; cap 10 MB)"
-                )
             mime_hint = resp.headers.get("content-type", "")
             name_hint = final_url
             source = final_url