full-day workshop: Correct the CI workflow triggers and first-run verification

Problem

content/full-day/4-testing.md describes merge as a GitHub Actions event, but there is no merge workflow event; a merge to main is normally handled as a push. Learners are also told to request an on-demand trigger, while the example omits workflow_dispatch.

The example restricts both push and pull_request to app/server/**. The first commit adds only .github/workflows/server-test.yml, so the text's claim that this push triggers the workflow is false: the path filter prevents that first run. Learners can reach the Actions page with no run to inspect.

Suggested change

Teach the actual event semantics and provide an example whose triggers support every verification step in the exercise.

Acceptance criteria

• merge is removed as an event name and explained as a push to the target branch.
• The example includes workflow_dispatch when on-demand execution is required.
• Adding or changing the workflow itself triggers a run, or learners are explicitly told to dispatch it manually.
• Server changes still trigger push and pull-request runs.
• The exact first-run flow is tested in a fresh template repository.

Related: #236 separately tracks the pytest/unittest inconsistency.

Start with content/full-day/4-testing.md and compare its trigger explanations with .github/workflows/server-test.yml. Verify the first-run flow in a fresh template repository, including workflow changes, server changes, pull requests, and on-demand execution. Done means the documented events and verification steps match the observed GitHub Actions runs.