fix(client): allow ClientSessionGroup to connect servers with no components (#3578)

[teddiesloco]

Description

Fixes #3578.

ClientSessionGroup._aggregate_components previously contained an exit-stack deletion check:

if not any((prompts_temp, resources_temp, tools_temp)):
    del self._session_exit_stacks[session]

This caused two issues:

1. When connecting via connect_with_session, the session was not registered in _session_exit_stacks, raising KeyError when connecting a server with zero components.
2. When connecting via connect_to_server, deleting the session from _session_exit_stacks prevented a subsequent disconnect_from_server from closing the session's exit stack, leaking the underlying transport until group teardown.

Servers without tools, prompts, or resources are valid under the MCP specification (e.g. servers providing only roots, sampling, or minimal utilities).

Changes

• Removed the unnecessary exit-stack deletion block in _aggregate_components.
• Added unit test test_client_session_group_connect_empty_components_server verifying that a server with no components connects without error and disconnects cleanly.

Testing

• uv run pytest tests/client/test_session_group.py passed (13 passed).
• uv run ruff check and uv run ruff format --check clean.

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3578.

If a maintainer assigns you to #3578, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.