refactor: use Client class in tests

[felixweinberger]

Refactors unit tests to use the ergonomic Client class instead of the verbose InMemoryTransport + ClientSession pattern.

Motivation and Context

Follow-up from #1870. The new Client class provides a simpler API for testing MCP servers:

# Before
transport = InMemoryTransport(server)
async with transport.connect() as (read_stream, write_stream):
    async with ClientSession(read_stream, write_stream) as session:
        await session.initialize()
        result = await session.list_tools()

# After
async with Client(server) as client:
    result = await client.list_tools()

How Has This Been Tested?

All refactored tests pass. The stream_spy fixture continues to work with Client since it patches the underlying memory streams used by InMemoryTransport.

Breaking Changes

None - internal test refactoring only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Files changed (5):

• tests/client/transports/test_memory.py - 3 tests
• tests/shared/test_session.py - 2 tests
• tests/server/test_cancel_handling.py - 1 test
• tests/shared/test_progress_notifications.py - 1 test
• tests/client/test_list_methods_cursor.py - 6 tests

Net change: -64 lines (163 added, 227 removed)