fix: use 2-space indentation in generated manifests YAML output

[Copilot]

score-implementation-sample generate was emitting YAML manifests with 4-space indentation instead of the standard 2-space.

Description

The generate command was calling yaml.NewEncoder(out).Encode(manifest) directly, which uses gopkg.in/yaml.v3's default 4-space indentation.

Fix: Create the encoder separately and call enc.SetIndent(2) before encoding:

// Before
_ = yaml.NewEncoder(out).Encode(manifest)

// After
enc := yaml.NewEncoder(out)
enc.SetIndent(2)
_ = enc.Encode(manifest)

Test expectations in generate_test.go updated to reflect 2-space indentation.

This mirrors the fix applied in score-spec/score-compose#461.

What does this PR do?

Fixes 4-space indentation in YAML manifest output from score-implementation-sample generate, aligning it with standard YAML formatting conventions.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• New chore (expected functionality to be implemented)

Checklist:

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

[mathieu-benoit]

LGTM