fix(attachments,usage): validate content behind a BOM, judge by the live parser table

Review follow-ups.

A BOM told the sniff which encoding to read, but was also taken as the
verdict: three prepended bytes let any binary through, including as
notes.txt. A BOM now only selects the test — UTF-8 falls through to the
byte rules on the remainder, UTF-16/32 decode and judge the characters
(NUL, unprintable, or replacement chars from bytes the decoder could not
read). Real Notepad-Unicode text still passes, mp4-behind-a-BOM does not,
in either language.

The gate treated the full parser table as a given, but without docling the
fallback extractor has no .tif/.tiff/.bmp/.webp/.vtt/.xml handler, so those
suffixes skipped the content check and reached the plain-text fallthrough —
the original bug, one install away. The worker now passes the keys of the
extractor it actually built, making the second gate stricter than the
route's static one rather than a copy of it.

Cache bins: extraction coerced a missing value to 0 and only non-zero bins
were recorded, so a provider reporting cached_tokens=0 persisted as NULL —
indistinguishable from "not reported", and OpenAI reports exactly that on
every uncached request. Bins are now carried as Optional and recorded when
not None, which is what the nullable columns and the NULL-means-unknown
comment already assumed. Anthropic's cache_read/cache_creation bins had the
same shape and are fixed alongside; the int-or-None coercion is shared in
llm/base.py.

tests/worker/test_attachment_worker.py

@@ -379,6 +379,53 @@ class TestAttachmentTypeGuard:
         failed = [payload for name, payload in events if name == "attachment.failed"]
         assert failed and failed[0]["error"] == "Unsupported file type: .mp4"
 
+    def test_suffix_the_loaded_extractor_cannot_parse_is_refused(
+        self, pg_conn, patch_worker_db, task_self, monkeypatch, tmp_path
+    ):
+        """The guard judges against the parser table actually loaded.
+
+        Without docling the fallback extractor has no .webp parser, so a
+        .webp the route admitted on its name would otherwise be opened as
+        plain text here.
+        """
+        from application import worker
+
+        local_path = tmp_path / "scan.webp"
+        local_path.write_bytes(b"RIFF\x00\x00\x00\x00WEBPVP8 " + bytes(range(256)))
+
+        events = []
+        fake_storage = MagicMock(name="storage")
+        fake_storage.process_file.side_effect = lambda path, callback: callback(
+            str(local_path)
+        )
+        monkeypatch.setattr(worker.StorageCreator, "get_storage", lambda: fake_storage)
+        # The docling-less fallback table: images are handled, .webp is not.
+        monkeypatch.setattr(
+            worker,
+            "get_default_file_extractor",
+            lambda ocr_enabled=False, pdf_text_fast_path=False: {".png": object()},
+        )
+        monkeypatch.setattr(
+            worker,
+            "publish_user_event",
+            lambda user, name, payload, **kwargs: events.append((name, payload)),
+        )
+
+        file_info = {
+            "filename": "scan.webp",
+            "attachment_id": "507f1f77bcf86cd799439014",
+            "path": "uploads/user1/attachments/scan.webp",
+            "metadata": {"source": "chat"},
+        }
+
+        with pytest.raises(
+            worker.AttachmentRejectedError, match=r"Unsupported file type: \.webp"
+        ):
+            worker.attachment_worker(task_self, file_info, "user1")
+
+        failed = [payload for name, payload in events if name == "attachment.failed"]
+        assert failed and failed[0]["error"] == "Unsupported file type: .webp"
+
     def test_text_without_a_parser_is_parsed(
         self, pg_conn, patch_worker_db, task_self, monkeypatch, tmp_path
     ):