fix(client): respect session group capabilities

[he-yufeng]

Fixes #2689.

ClientSessionGroup was querying prompts, resources, and tools on every connection, even when the server did not advertise those capabilities during initialize. For a tools-only server that meant two extra JSON-RPC calls that came back as Method not found warnings.

This changes _aggregate_components to use negotiated capabilities when an initialize_result is available:

• skip list_prompts() when capabilities.prompts is absent
• skip list_resources() when capabilities.resources is absent
• skip list_tools() when capabilities.tools is absent
• preserve the existing fallback behavior when no initialize result is available

I also added a regression test for a tools-only server session: it still aggregates the advertised tool, and list_prompts / list_resources are not awaited.

Validation:

uv run python -m py_compile src/mcp/client/session_group.py tests/client/test_session_group.py
uv run ruff check src/mcp/client/session_group.py tests/client/test_session_group.py
uv run ruff format --check src/mcp/client/session_group.py tests/client/test_session_group.py
uv run pyright src/mcp/client/session_group.py tests/client/test_session_group.py
uv run pytest tests/client/test_session_group.py -q