Fix stale MCP tools after disconnect

[KXHXK]

What does this PR do?

Fixes #2569.

MCPClient.disconnect() now clears the session's tool list after exiting the adapter. This prevents get_tools() from returning tools bound to a closed MCP connection and restores its documented behavior of raising until a new session is connected.

The cleanup runs in finally, so stale tools are also removed when the adapter raises during shutdown. The original shutdown exception still propagates unchanged.

Testing

• Regression-first: 2 tests failed before the implementation
• tests/test_mcp_client.py: 7 passed
• TestToolCollection::test_from_mcp: 1 passed
• Ruff lint and format checks: passed
• git diff --check: passed
• Secret-pattern scan of changed files: passed

AI assistance: Codex helped identify the issue, implement the change, and run validation; the contribution was reviewed and submitted by KXH.