feat: stream shorebird_tests output for CI debugging

[eseidel]

Summary

• Switches _runFlutterCommand from Process.run (buffered) to Process.start (streaming) so stdout/stderr are printed in real time
• Helps diagnose flaky timeout failures in android_test.dart on CI — if a process hangs, the logs will show exactly where it stopped

Test plan

• Verify shorebird_tests CI job now shows streaming output for each flutter command

[eseidel]

Example failure: https://github.com/shorebirdtech/flutter/actions/runs/21611184849/job/62280081866

Before I try fixes I want to see full logs, which this should give us.

[eseidel]

The log format I (claude) picked is probably too verbose though, I may remove the prefix.