[Fleet] Fix root level variable replacement yaml migration

[nchaulet]

Summary

Migrating to yaml from js-yaml in #252345 break some agent template features, notably replacing a root yaml variable with quote inside, that PR aim to fix it

Resolve elastic/integrations#18739

Test

I added unit test to cover that change

You can manually try to create a syslog_router package policy and verify it's working as expected

[infra-vault-gh-plugin-prod]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

@elasticmachine merge upstream

[macroscopeapp]

Approvability

Verdict: Needs human review

This PR changes the order of YAML processing operations and modifies stringify behavior in Fleet's template compilation. The changed files are owned by @elastic/fleet, and designated code owners should review changes to their YAML processing logic.

^{You can customize Macroscope's approvability policy. Learn more.}

[jen-huang]

LGTM 🚀

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 963a45f

Failed CI Steps

• FTR Configs #68

Metrics [docs]

✅ unchanged

cc @nchaulet