fix(client): allow ClientSessionGroup to connect to servers without components

[Alphaxiaoteng]

Summary

Fixes #3384.

Connecting a ClientSessionGroup to an MCP server that legitimately exposes no tools, resources, or prompts previously triggered a del self._session_exit_stacks[session] in _aggregate_components.

This caused two issues:

1. When connected via connect_with_session(), the session exit stack was never inserted into self._session_exit_stacks, leading to an uncaught KeyError.
2. When connected via connect_to_server(), the stack was removed without closing or tracking, leaving the session registered in self._sessions but with no cleanup path on disconnect_from_server().

Changes

• Removed the incorrect del self._session_exit_stacks[session] cleanup in _aggregate_components. Empty component sets are valid and properly recorded as empty component sets for the connected session.
• Added regression test test_client_session_group_connect_empty_server_succeeds in tests/client/test_session_group.py.

Verification

• uv run pytest tests/client/test_session_group.py (13 passed).
• uv run ruff check src/ tests/ (passed).
• uv run ruff format --check tests/client/test_session_group.py (passed).
• uv run pyright (0 errors).

[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because you aren't currently assigned to #3384.

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