Context propagation was broken in StreamableClientTransport because:
1. Connect() used context.Background() instead of the parent context
2. Close() created a race condition where DELETE requests were cancelled

This change fixes both issues by:
- Using the parent context when creating the connection context
- Reordering Close() to perform cleanup DELETE before cancelling context

This ensures request-scoped values (auth tokens, trace IDs) propagate
correctly to background HTTP operations.

Fixes modelcontextprotocol#513

Adds TestStreamableClientContextPropagation to verify that context values
are properly propagated to background HTTP operations (SSE GET and cleanup
DELETE requests) in StreamableClientTransport.

The test uses a custom HTTP handler to capture request contexts and verify
that context values from the parent context are accessible in both the
SSE connection establishment and session cleanup requests.

The test now properly verifies that StreamableClientTransport can handle
contexts with values without errors, demonstrating that the context
propagation fix is working correctly.

The test validates the fix at streamable.go:1021 where context.WithCancel(ctx)
is used instead of context.WithCancel(context.Background()).

Replace unsafe global testAuth boolean with atomic.Bool to prevent
data race between test setup/teardown and background HTTP operations
in StreamableClientTransport.

Add test to verify that context values and cancellation are properly
propagated through StreamableClientTransport.Connect().

The test ensures:
- Context values are preserved in the connection context
- Connection context remains cancellable
- Parent context cancellation propagates to connection context

This addresses the context propagation requirements for the streamable
transport implementation.