ci: run tests on main pushes

[danReynolds]

Summary

Adds a push trigger for main to the Flutter test workflow, while keeping the existing pull_request trigger for PRs targeting main.

Why

The workflow currently runs for PR updates only. After a PR is merged, the resulting commit on main does not start CI, so merge commits can land without a post-merge verification run.

Verification

• git diff --check

[Copilot]

Pull request overview

Updates the Flutter test GitHub Actions workflow so CI runs not only for PRs targeting main, but also for commits pushed to main (e.g., after merges), ensuring post-merge verification.

Changes:

• Add a push trigger for the main branch while keeping the existing pull_request trigger for PRs targeting main.
• Minor YAML cleanup/formatting in the workflow file.

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