test: unit test flakes for activity logs stream

Summary

A recent test run on main failed for reasons unrelated to the recent commit. Perhaps a timing of a poll?

The retry succeeded! 🧪 ✨

Preview

🔗 https://github.com/slackapi/slack-cli/actions/runs/23178071163/job/67344594805

=== RUN   TestPlatformActivity_StreamingLogs/should_return_nil_if_TailArg_is_set_and_activity_request_fails_while_polling
    activity_test.go:267: 
        	Error Trace:	/Users/runner/work/slack-cli/slack-cli/internal/pkg/platform/activity_test.go:267
        	            				/Users/runner/work/slack-cli/slack-cli/internal/pkg/platform/activity_test.go:296
        	Error:      	Not equal: 
        	            	expected: 3
        	            	actual  : 4
        	Test:       	TestPlatformActivity_StreamingLogs/should_return_nil_if_TailArg_is_set_and_activity_request_fails_while_polling
        	Messages:   	Expected number of calls (3) of method Activity does not match the actual number of calls (4).

https://github.com/slackapi/slack-cli/blob/f23e6d3546f93e8fba8f384f89c3b77dacb34934/internal/pkg/platform/activity_test.go#L249-L269

Start with internal/pkg/platform/activity_test.go around lines 249-269 and run the TestPlatformActivity_StreamingLogs case from the linked Actions failure. Investigate the polling timing behind the mismatch between the expected three Activity calls and the actual four, then verify the test reliably covers the request-failure behavior without flaking.