feat(syslog): list patterns and add a ready-made one through MCP

Patterns could only be added on the Syslog page, so an agent asked to map
them on the NASes couldn't. list_syslog_patterns gives each pattern's metric
and the ready-made ones not added; add_syslog_preset (admin, audited) adds one.
The key-picking helper moves beside the patterns for both callers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_syslog.py

@@ -290,6 +290,38 @@ def test_a_source_page_filters_its_lines_and_hosts_show_them(app):
     asyncio.run(go())
 
 
+def test_a_preset_is_added_once_through_mcp(app):
+    from sqlalchemy import delete
+
+    from steward.core.api_tokens import Caller
+    from steward.mcp import _context
+    from steward.models.users import UserRole
+
+    tools = _mod("mcp_tools")
+    SyslogPattern = _mod("models").SyslogPattern
+
+    async def go():
+        _context._app_ctx.set(app)
+        _context._caller_ctx.set(Caller(user_id=None, username="admin", role=UserRole.admin, token_id=0))
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                await s.execute(delete(SyslogPattern).where(SyslogPattern.key == "nfs"))
+        with pytest.raises(ValueError, match="No preset"):
+            await tools.add_syslog_preset("no-such-preset")
+        added = await tools.add_syslog_preset("nfs")
+        assert added["metric"] == "lines_matching_nfs"
+        listed = await tools.list_syslog_patterns()
+        assert "nfs" in {p["key"] for p in listed["patterns"]}
+        assert "nfs" not in {p["key"] for p in listed["presets_not_added"]}
+        with pytest.raises(ValueError, match="already added"):
+            await tools.add_syslog_preset("nfs")
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                await s.execute(delete(SyslogPattern).where(SyslogPattern.key == "nfs"))
+
+    asyncio.run(go())
+
+
 def test_search_syslog_filters_and_pages(app):
     from steward.core.api_tokens import Caller
     from steward.mcp import _context