fix: preserve camelCase metadata in machine-readable summaries

[immanuwell]

What?

Fix machine-readable summary metadata keys.

Repro:
printf 'export default function() {}\n' > script.js
k6 run --summary-export=summary.json --new-machine-readable-summary=true script.js

Before:
metadata.generated_at
metadata.k6_version

After:
metadata.generatedAt
metadata.k6Version

Why?

This is easy to hit on a plain local run, no cloud bits or quotas involved.
The new summary was going through JS as Go structs, so Sobek rewrote the field names to snake_case. Bit sneaky.
This switches that path to JSON-compatible maps and adds tests for handleSummary(data) and CLI export.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

• metadata.k6_version in summary.json should be k6Version when producing machine-readable-summary #6004
• Machine readable end of test summary #4803

Closes #6004