fix(api): guard against missing session in get_agent_session (#15011)

`GET /agents/<agent_id>/sessions/<session_id>` crashed with
`AttributeError: 'NoneType' object has no attribute 'to_dict'` when the
session lookup failed: `_, conv =
API4ConversationService.get_by_id(...)` returned `(False, None)`, then
`conv.to_dict()` was called unconditionally.

This is reachable in multi-instance deployments: the session row may not
yet be visible on the node servicing the immediate follow-up GET after a
session is created on a different node.

Add the same `if not exists` guard already used by every other call site
of `API4ConversationService.get_by_id` (see agent_api.py:1147,
sdk/session.py:179, conversation_service.py:248, canvas_service.py:323).

Closes #14989

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

test/unit_test/api/apps/restful_apis/test_get_agent_session.py

@@ -0,0 +1,129 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+"""Regression tests for `get_agent_session` (api/apps/restful_apis/agent_api.py)."""
+
+import importlib.util
+import sys
+from pathlib import Path
+from types import ModuleType, SimpleNamespace
+
+import pytest
+
+
+class _PassthroughManager:
+    def route(self, *_args, **_kwargs):
+        return lambda func: func
+
+
+def _stub(monkeypatch, name, **attrs):
+    mod = ModuleType(name)
+    for key, value in attrs.items():
+        setattr(mod, key, value)
+    monkeypatch.setitem(sys.modules, name, mod)
+    return mod
+
+
+def _load_agent_api(monkeypatch, get_by_id_result):
+    """Load api/apps/restful_apis/agent_api.py with the minimum stubs required.
+
+    `get_by_id_result` is the `(exists, conv)` tuple the stub
+    `API4ConversationService.get_by_id` will return for any session_id.
+    """
+    _stub(monkeypatch, "api.apps", current_user=SimpleNamespace(id="tenant-1"), login_required=lambda func: func)
+    _stub(monkeypatch, "api.apps.services.canvas_replica_service", CanvasReplicaService=SimpleNamespace())
+    _stub(monkeypatch, "api.db", CanvasCategory=SimpleNamespace())
+    _stub(monkeypatch, "api.db.db_models", Task=SimpleNamespace())
+    _stub(
+        monkeypatch,
+        "api.db.services.api_service",
+        API4ConversationService=SimpleNamespace(get_by_id=lambda _session_id: get_by_id_result, save=lambda **_kwargs: True, delete_by_id=lambda *_args, **_kwargs: True, query=lambda **_kwargs: []),
+    )
+    _stub(
+        monkeypatch,
+        "api.db.services.canvas_service",
+        CanvasTemplateService=SimpleNamespace(),
+        UserCanvasService=SimpleNamespace(accessible=lambda *_a, **_k: True, query=lambda **_kwargs: []),
+        completion=lambda *_a, **_k: None,
+        completion_openai=lambda *_a, **_k: None,
+    )
+    _stub(monkeypatch, "api.db.services.document_service", DocumentService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.file_service", FileService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.knowledgebase_service", KnowledgebaseService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.pipeline_operation_log_service", PipelineOperationLogService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.task_service", CANVAS_DEBUG_DOC_ID="", TaskService=SimpleNamespace(), queue_dataflow=lambda *_a, **_k: None)
+    _stub(monkeypatch, "api.db.services.user_service", TenantService=SimpleNamespace(), UserService=SimpleNamespace(get_by_id=lambda *_a, **_k: (False, None)))
+    _stub(monkeypatch, "api.db.services.user_canvas_version", UserCanvasVersionService=SimpleNamespace())
+    _stub(
+        monkeypatch,
+        "api.utils.api_utils",
+        add_tenant_id_to_kwargs=lambda func: func,
+        get_data_error_result=lambda message="Sorry": {"code": 102, "message": message, "data": None},
+        get_json_result=lambda code=0, message="", data=None: {"code": code, "message": message, "data": data},
+        get_result=lambda **kwargs: kwargs,
+        get_request_json=lambda: {},
+        server_error_response=lambda exc: {"code": 500, "message": str(exc)},
+        validate_request=lambda *_a, **_k: lambda func: func,
+    )
+    _stub(monkeypatch, "common.settings", retriever=SimpleNamespace(), kg_retriever=SimpleNamespace())
+    _stub(monkeypatch, "common.ssrf_guard", assert_host_is_safe=lambda *_a, **_k: None)
+
+    repo_root = Path(__file__).resolve().parents[5]
+    module_path = repo_root / "api" / "apps" / "restful_apis" / "agent_api.py"
+    spec = importlib.util.spec_from_file_location("test_get_agent_session_agent_api", module_path)
+    module = importlib.util.module_from_spec(spec)
+    module.manager = _PassthroughManager()
+    monkeypatch.setitem(sys.modules, "test_get_agent_session_agent_api", module)
+    spec.loader.exec_module(module)
+    return module
+
+
+@pytest.mark.p1
+class TestGetAgentSession:
+    """Regression for #14989: GET /agents/<id>/sessions/<sid> must not crash
+    with `AttributeError: 'NoneType' object has no attribute 'to_dict'` when
+    the session_id does not exist."""
+
+    @pytest.mark.p1
+    def test_returns_error_when_session_missing(self, monkeypatch):
+        """Missing session must return a data-error JSON, not raise AttributeError.
+
+        In multi-instance deployments, the session row may not yet be visible
+        on the node servicing the GET. The previous implementation called
+        `conv.to_dict()` on the `None` returned by `get_by_id` and crashed.
+        """
+        module = _load_agent_api(monkeypatch, get_by_id_result=(False, None))
+
+        result = module.get_agent_session(agent_id="agent-1", session_id="does-not-exist", tenant_id="tenant-1")
+
+        assert result == {
+            "code": 102,
+            "message": "Session not found!",
+            "data": None,
+        }
+
+    @pytest.mark.p1
+    def test_returns_session_dict_when_found(self, monkeypatch):
+        """When the session exists, the route returns its `to_dict()` payload."""
+        conv = SimpleNamespace(to_dict=lambda: {"id": "sess-1", "messages": []})
+        module = _load_agent_api(monkeypatch, get_by_id_result=(True, conv))
+
+        result = module.get_agent_session(agent_id="agent-1", session_id="sess-1", tenant_id="tenant-1")
+
+        assert result == {
+            "code": 0,
+            "message": "",
+            "data": {"id": "sess-1", "messages": []},
+        }