[feature]Implement session cache for Doris connections (#44)

* [feature]Implement session cache for Doris connections

This PR introduces a `DorisSessionCache` to cache and reuse `DorisConnection` objects in memory.
This helps to reduce the overhead of creating new connections, especially for frequently used system sessions like "query"
and "system", and avoid not calling release_connection leads to `Connection acquisition timed out` when the number of
connection pools reaches the maximum value.

The PR #34 fixed the issue when calling the tool `exec_query`, but in the codebase, a large number of other tools directly
using get_connection ("query") to get connection object but without calling the release_connection method will cause the
connection to fail to be obtained after a certain number of times.

Key changes:
- Added `DorisSessionCache` class to manage the lifecycle of cached sessions.
- The cache is configurable to store system sessions, user sessions, or both. By default, only system sessions are cached.
- Integrated the session cache into `DorisConnectionManager`.
- `get_connection` now checks the cache before creating a new connection.
- `release_connection` removes the connection from the cache.

* Add tests

test/utils/test_db.py

@@ -0,0 +1,78 @@
+from unittest.mock import MagicMock
+import pytest
+
+from doris_mcp_server.utils.db import DorisConnection, DorisSessionCache
+
+
+@pytest.fixture
+def session_cache():
+    """Provides a DorisSessionCache instance with a mock connection manager."""
+    connection_manager = MagicMock()
+    cache = DorisSessionCache(connection_manager=connection_manager)
+    yield cache, connection_manager
+
+
+class TestDorisSessionCache:
+
+    def test_initialization(self, session_cache):
+        cache, _ = session_cache
+        assert cache.cache_system_session is True
+        assert cache.cache_user_session is False
+        assert not cache.cached
+
+    def test_should_cache(self, session_cache):
+        cache, _ = session_cache
+        assert cache._should_cache("query") is True
+        assert cache._should_cache("system") is True
+        assert cache._should_cache("user-test-session-id") is False
+
+        cache.cache_user_session = True
+        assert cache._should_cache("user-test-session-id") is True
+
+    def test_save_and_get_session(self, session_cache):
+        cache, _ = session_cache
+        mock_connection = MagicMock(spec=DorisConnection)
+        mock_connection.session_id = "query"
+
+        cache.save(mock_connection)
+        retrieved_conn = cache.get("query")
+        assert retrieved_conn is mock_connection
+
+        mock_user_connection = MagicMock(spec=DorisConnection)
+        mock_user_connection.session_id = "user-test-session-id"
+        cache.save(mock_user_connection)
+        assert cache.get("user-test-session-id") is None
+
+        cache.cache_user_session = True
+        cache.save(mock_user_connection)
+        retrieved_user_conn = cache.get("user-test-session-id")
+        assert retrieved_user_conn is mock_user_connection
+
+    def test_remove_session(self, session_cache):
+        cache, _ = session_cache
+        mock_connection = MagicMock(spec=DorisConnection)
+        mock_connection.session_id = "system"
+
+        cache.save(mock_connection)
+        assert cache.get("system") is not None
+
+        cache.remove("system")
+        assert cache.get("system") is None
+
+    def test_clear_cache(self, session_cache):
+        cache, connection_manager = session_cache
+        mock_conn1 = MagicMock(spec=DorisConnection)
+        mock_conn1.session_id = "query"
+        mock_conn2 = MagicMock(spec=DorisConnection)
+        mock_conn2.session_id = "system"
+
+        cache.save(mock_conn1)
+        cache.save(mock_conn2)
+        assert len(cache.cached) == 2
+
+        cache.clear()
+
+        assert not cache.cached
+        connection_manager.release_connection.assert_any_call("query", mock_conn1)
+        connection_manager.release_connection.assert_any_call("system", mock_conn2)
+        assert connection_manager.release_connection.call_count == 2