fix(gateway): remove app-specific Athabasca references from vision enrichment (#1529)

Salvaged from PR #1428 by jplew.

Removes Athabasca-specific persistence guidance accidentally merged
in PR #1422:
- Drop Athabasca docstring and injected note from _enrich_message_with_vision
- Delete tests/gateway/test_image_enrichment.py (asserted app-specific behavior)

Co-authored-by: jplew <jplew@users.noreply.github.com>

tests/gateway/test_image_enrichment.py

@@ -1,25 +0,0 @@
-from unittest.mock import patch
-
-import pytest
-
-
-@pytest.mark.asyncio
-async def test_image_enrichment_uses_athabasca_upload_guidance_without_stale_r2_warning():
-    from gateway.run import GatewayRunner
-
-    runner = object.__new__(GatewayRunner)
-
-    with patch(
-        "tools.vision_tools.vision_analyze_tool",
-        return_value='{"success": true, "analysis": "A painted serpent warrior."}',
-    ):
-        enriched = await runner._enrich_message_with_vision(
-            "caption",
-            ["/tmp/test.jpg"],
-        )
-
-    assert "R2 not configured" not in enriched
-    assert "Gateway media URL available for reference" not in enriched
-    assert "POST /api/uploads" in enriched
-    assert "Do not store the local cache path" in enriched
-    assert "caption" in enriched