Harden closed-advisory fixes (#16409)

## Summary
- harden reopened advisory fixes across REST connector, invoke, document
downloads, and markdown rendering
- add targeted regression coverage for redirect-safe SSRF handling,
invoke SSRF checks, document access control, and markdown sanitization
- verify each referenced GHSA against the original GitHub advisory text
and align the closed-advisory plan with the implemented remediation

## What changed
- add tenant access checks to document download endpoints to avoid
cross-tenant document disclosure
- add per-hop SSRF validation, DNS pinning, redirect handling, and
redirect limits to the REST API connector
- ensure invoke requests validate and pin the resolved host and never
follow redirects implicitly
- keep the generic rate-limited request path wrapped, not just GET and
POST helpers
- sanitize markdown HTML before rendering in the highlight markdown
component

## Validation
- `cd web && npm test -- --runInBand
src/components/highlight-markdown/__tests__/index.test.tsx`
- `.venv/bin/python -m pytest -q
test/unit_test/data_source/test_rest_api_connector.py`
- targeted `test/testcases/test_web_api/...` unit additions were
reviewed, but the suite cannot be executed end-to-end in this
environment because parent `test/testcases/conftest.py` requires a local
service on `127.0.0.1:9380`

## Notes
- all GHSA entries referenced by the plan were checked against the
original GitHub advisory text, not sampled
- the closed-advisory plan document was updated locally during review,
but is intentionally not included in this PR

test/testcases/test_web_api/test_document_app/test_document_metadata.py

@@ -450,6 +450,23 @@ class TestDocumentMetadataUnit:
         assert res["code"] == 500
         assert "download boom" in res["message"]
 
+    def test_download_document_rejects_other_tenant_unit(self, document_rest_api_module, monkeypatch):
+        module = document_rest_api_module
+        monkeypatch.setattr(module.DocumentService, "accessible", lambda _doc_id, _user_id: False)
+
+        res = _run(module.download_document("doc1"))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert "Document not found!" in res["message"]
+
+    def test_dataset_document_download_rejects_other_tenant_unit(self, document_rest_api_module, monkeypatch):
+        module = document_rest_api_module
+        monkeypatch.setattr(module.KnowledgebaseService, "accessible", lambda kb_id, user_id: False)
+        monkeypatch.setattr(module.DocumentService, "accessible", lambda _doc_id, _user_id: True)
+
+        res = _run(module.download("kb1", "doc1"))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert "Document not found!" in res["message"]
+
     @pytest.mark.p2
     def test_get_document_image_content_type_from_object_extension_unit(self, document_app_module, monkeypatch):
         module = document_app_module