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/restful_api/test_document_raw_routes.py

@@ -45,7 +45,7 @@ def test_document_download_by_id_invalid_id_contract(rest_client):
     assert res.status_code == 200
     payload = res.json()
     assert payload["code"] == 102, payload
-    assert payload["message"] == "The dataset not own the document invalid_document_id.", payload
+    assert payload["message"] == "Document not found!", payload
 
 
 @pytest.mark.p2

test/testcases/restful_api/test_documents.py

@@ -1503,14 +1503,14 @@ def test_documents_download_requires_auth_and_invalid_id_contract(rest_client, c
     assert invalid_doc_res.status_code == 200
     invalid_doc_payload = invalid_doc_res.json()
     assert invalid_doc_payload["code"] == 102, invalid_doc_payload
-    assert "The dataset not own the document invalid_document_id." in invalid_doc_payload["message"], invalid_doc_payload
+    assert invalid_doc_payload["message"] == "Document not found!", invalid_doc_payload
 
     invalid_dataset_path = tmp_path / "invalid_dataset_download.txt"
     invalid_dataset_res = _download_document_to_file(rest_client, "invalid_dataset_id", document_id, invalid_dataset_path)
     assert invalid_dataset_res.status_code == 200
     invalid_dataset_payload = invalid_dataset_res.json()
     assert invalid_dataset_payload["code"] == 102, invalid_dataset_payload
-    assert f"The dataset not own the document {document_id}." in invalid_dataset_payload["message"], invalid_dataset_payload
+    assert invalid_dataset_payload["message"] == "Document not found!", invalid_dataset_payload
 
 
 @pytest.mark.p2