init: update the default sample Score file generated

[Abhishek9639]

Description

The default score.yaml generated by score-helm init was using a basic stefanprodan/podinfo example. This PR replaces it with a more meaningful sample for consistency across all Score implementations.

What does this PR do?

Follow-up to score-spec/score-compose#476, score-spec/score-k8s#302, and score-spec/score-implementation-sample#103 as requested by @mathieu-benoit.

Updates the sample to use scorespec/sample-score-app:latest with postgres, dns, and route resources. Switched from struct-based YAML generation to a DefaultScoreFileContent raw string constant, which is simpler and supports comments.

Updated TestInitNominal and TestInitAndGenerate_with_sample accordingly.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I've signed off with an email address that matches the commit author.

[Abhishek9639]

Hi @mathieu-benoit,
This is the last one score-helm is now updated too. All four repos (score-compose, score-k8s, score-implementation-sample, and score-helm) should now be consistent.
Please take a look when you get a chance.
Thanks

[mathieu-benoit]

LGTM